Here is a list I've gleaned from elsewhere of when it is known two or more Beatles were in the same place.
1970January - George, Paul and Ringo complete various overdubs for the forthcoming “Let it Be” album. John is notably absent.
March 1970 – Pattie Boyd’s party at Friar Park. Chris O'Dell claims this was the last time all four Beatles were together but other sources say he is mistaken and Paul or John (even both) did not attend.
Ringo plays drums on George’s first solo outing “All Things Must Pass”
1971George returns the favour by writing and playing on Ringo’s single. “It Don’t Come Easy” in 1971.
George plays on the majority of Lennon’s “Imagine” album although not the title track.
Ringo also plays with George at the Concert for Bangladesh in 1971 in New York. George invited both Paul and John. Paul declined as he felt he wasn’t ready for a Beatle reunion. John at first accepted and then realizing that Yoko was not welcome snubbed the event.
1972George plays on Ringo’s hit single “Back Off Boogaloo”
1973January 13- George and Ringo perform at Eric Claption’s concert at the Rainbow London.
Ringo plays drums on George’s “Living in the Material World”.
All 4 Beatles play on and write songs for Ringo’s self titled album. A Beatles re-union of sorts although there is no single track with all four Beatles playing on it.
1974John plays on the title track Ringo album “Goodnight Vienna”. Ringo drums on two tracks for George’s Dark Horse Album.
Last known photo of John and Paul circa 1974March - Ringo and John begin to hang out together in LA. Paul and John jam together in LA whilst John is producing the Harry Nilsson album “Pussycats” in a totally forgettable session. Plans for Lennon to record on some tracks on McCartney’s next album “Venus and Mars” in New Orleans are talked of.
Last known photo of George and John in 1974George and John appear on a radio station together - Radio KHJ 930 AM on 21st December 1974. This may well be the last time George met John in person.
19th Dec 1974 A pre-arranged meeting in the Plaza Hotel, New York for the four Beatles to sign the terms for dissolution of The Beatles earlier in the year was snubbed by John because the stars weren’t right, much to the annoyance of the other Beatles (although Ringo had remained in England after signing the papers earlier).
They were finally signed, and the Beatles dissolved by John, at the Polynesian Resort at Walt Disney World, Florida. Perhaps because he started the band and wanted to be the one who finally ended it.
1975March 24th, Paul throws a launch party for “Venus & Mars” aboard the Queen Mary, permanently moored at Long Beach Harbor, to celebrate the LP's completion. Among those in attendance was George Harrison.
1976John and Paul record (separately) on 3 tracks for Ringo’s Rotogravure album. John’s last studio recording is on June 12th 1976 before his retirement until 1980.
On April 24, 1976 Paul visits John in the Dakota and they both watch Saturday Night Live together. An offer for $3000 is made for the Beatles to come down and perform. They actually consider it for a moment but decide they are too tired.
April 25- Paul McCartney visits John Lennon at Dakota Apartments in New York City, New York. Paul brought along his guitar, arriving unexpectedly. John threw him out and it was probably the last time Paul saw John
May 9- George Harrison and Ringo Starr attended Paul McCartney and Wings concert at Maple Leaf Gardens in Toronto, Canada
Drum and Bass in 1976
Ringo joins Paul on stage at the end of the LA show on June 18, 1976 during his Wings Over America tour.
1977Date unknown. Allegedly the last time Paul sees John as they visit a French Restaurant together in New York. However this is contradicted by the Lennon Playboy interview of Sept 1980 and the McCartneys in 1984.
1979May 19th George, Paul and Ringo all perform together at Eric Clapton’s wedding and charge bystanders a 4 figure sum for photographs with the the Fab three. Apparently John Lennon was not invited although afterwards was, for the first time in years, proud of his former band mates for such a brazen stunt and lamented not being there according to one of the members of this board
Also in 1979 Paul McCartney set the wheels in motion for a Beatles reunion with a highly lucrative clause in a record label contract signed in 1979. The £6 million contract with CBS was not made public until 2005.The terms of the contract stipulated that McCartney must be allowed to record under the name 'The Beatles'.Lennon followed a year later with similar overtures to a Beatles reunion.
At the very end of the years rumours grow that the Beatles will reform and play at the Concert for Kampuchea after “Special Guests” are advertised on the bill. In the end only one Beatle - Paul and Wings turn up for their last concert.
1980From "The Beatles After the Break Up” by Keith Badman.
This is one of the most controversial pieces of minutiae of the Beatles story as Badman has been unable to verify his sources to our very own John S.Damn in years gone by on these boards. The Pythons did play the Hollywood Bowl during this time slot as it is documented on film. I recently watched a TV documentary about the Pythons talking about George and how he'd participated in the "Lumber Jack Song" with them but even though they talked about the the 1980 Hollywood Bowl shows there was no further mention of George (and John) being at the show.
After Lennon’s death George stated it had been a couple of years (not months) since he had last seen John Lennon. Badman claimed the story came from an American newspaper (and/or possibly a couple of fanzines). He believes the meeting was very brief and perhaps little more than pleasantries were exchanged. However the story has never been proven to be true.
This is the last known photo of John with any of the other Beatles.A year later at a much happier event.
1981Ringo and Paul play on and add vocals to George’s tribute to John “All Those Years Ago”. Paul and George write and play on Ringo’s “Stop and Smell the Roses album” although not together. Ringo declines to use the two John Lennon tracks that Lennon had written for the album, "Nobody Told Me" and "Life Begins At 40"
April 27- Paul and Linda McCartney, George and Olivia Harrison attended Ringo Starr’s wedding to Barbara Bach
1981 – 1984Ringo and George Martin appear in the promo video for Paul’s Take It Away Single.
Ringo plays on the next three of Paul’s solo albums “Tug of War”, “Pipes of Peace” and “Give My Regards to Broad Street”. He also appears alongside Paul in the much maligned “Broad Street” movie.
1987Ringo plays on George’s album Cloud Nine and can be seen in the video for “When We Was Fab” alongside a bass player disguised as a walrus (who appears briefly with George and Ringo about 3 minutes in). One story is that Paul McCartney was keen to be in the video but wanted to keep stories of a Beatle reunion quiet and so dressed as the Walrus.
Although the official story is that Paul could not fit time in his calendar to work with George and Ringo there is some evidence to suggest it was all another case of the Beatles fooling us. Check out this interview of Paul on kids show Going Live from 1987. About 7:30 into the video he talks about being in one of George’s videos as a Walrus!
1988
March 5th - George and Ringo appear together on the Micheal Aspel show in the UK during a humorous and enjoyable interview. Some shocked and stunned moments in there as well.
January 20th - George Harrison, Ringo Starr, Barbara Bach, Yoko Ono, Julian Lennon, and Sean Lennon attended the Beatles’ induction into the Rock n Roll Hall of Fame at the Waldorf Astoria Hotel in New York City, New York. Paul does not show due to business differences.
1993 -1996All three surviving Beatles work together on the music and then promoting the Beatles Anthology. Free As A Bird and Real Love are released as “new” Beatles recordings. However no 3rd new Beatles track is added to Anthology 3.
1997Ringo adds drums to Paul’s “Flaming Pie” album and co-writes “Really Love You”.
1998Paul and George play on Ringo’s “Vertical Man” album.
A Lennon demo recording, “Grow Old With Me” from the Dakota days is spruced up by George Martin for the Lennon Anthology boxed set with overdubs. Rumoured to have been one of the demos handed to the surviving Beatles for the Anthology project a very McCartney like bass line can be heard on the track despite remaining uncredited on the sleeve notes.
2000Paul and George travel to see the Cirque de Soleil Show in Las Vegas in preparation for the Beatles “Love” project. George sadly never sees the show come to fruition.
November 29th 2001 – George passes away peacefully. It is reported that both Ringo and Paul visited him together to say their goodbyes in his hospital room in Staten Island, NY in late Oct. 2001.
One of the last known pics of Paul, Ringo and George
2003Paul and Ringo appear together in the Concert for George along with many others.
2005
Ringo is snubbed for the opening of Live 8 and does not play with McCartney and U2.
2009Ringo and Paul both play live together at a fundraiser for the David Lynch Foundation.
2010Paul appears on Ringo’s album “Y Not” and also at the end of Ringo’s 70th Birthday Concert on July 7th
2011 June 18th Paul McCartney's Birthday. Ringo is playing live at the Empire Theatre in Liverpool. Paul is at LIPA a short distance away. Does Paul join Ringo on stage in their hometown to rapturous applause? No.
2012 Feb 6th Paul and Ringo attend Dhani Harrison’s wedding.
2014Feb 9th - Paul and Ringo (The Beatles Drum & Bass) perform for the 50th anniversary of appearing on the Ed Sullivan Show.
I might have got some of those dates wrong and probably missed a few meetings as well. So please feel free to correct anything.
One thing I can''t find is a gathering of all four Beatles once the band had split up. I am guessing it probably never happened.