As the Beatles' split was very much 3 vs. 1, it's not surprising that all of the group but Paul continued to work together throughout 1970, particularly once John was back in the country in September.
George had spent all summer working with Phil Spector and dozens of musicians, including Ringo, on All Things Must Pass. He also produced and/or played on sessions for Billy Preston, Doris Troy, and Ashton, Gardner & Dyke. Ringo drummed on Stephen Stills's solo album, and recorded a single of his own, "It Don't Come Easy"/"Early 1970", with help from George and Stephen.
At the end of September, John began recording his own solo album, John Lennon/Plastic Ono Band at EMI. Behind the drum kit was Ringo, and behind the mixing desk for the latter part of the sessions was Phil Spector. When John turned 30 on October 9th, 1970, he was presented with tapes featuring birthday jingles and greetings from other artists, such as Donovan, Janis Joplin, Nina Simone, and Blossom Dearie.
George and the ATMP crew had recorded their own tribute, "It's Johnny's Birthday", the tune lifted wholesale from Cliff Richard's hit "Congratulations". And Ringo and company (including Stills, Preston, and POB bassist Klaus Voormann) refashioned "Johnny B. Goode" into "Happy Birthday John".
This spirit of camaraderie is evident on a recording from the night of October 9th, take four of "Remember", where John is having a blast recording with Ringo and Klaus; when the take is complete, George walks into the studio (he was next door mixing ATMP) and is greeted heartily by all present.