Background: "Free as a Bird" is a song originally composed and recorded in 1977 as a home demo by John Lennon. In 1995, a studio version of the recording, incorporating contributions from Paul McCartney, George Harrison and Ringo Starr, was released as a single by the Beatles. It was released 25 years after the break-up of the band and 15 years after the death of Lennon. The single was released as part of the promotion for The Beatles Anthology video documentary and the band's Anthology 1 compilation album.
Context: George Martin, who had produced most of the Beatles' 1960s recordings, turned down an invitation to produce "Free as a Bird" due to hearing problems (though he subsequently managed to produce and direct the Anthology series). Harrison, in turn, suggested Lynne as producer (co-producer of his 1987 album, Cloud Nine) and work commenced at McCartney's studio in February 1994. Geoff Emerick and Jon Jacobs were chosen to engineer the new tracks.  The original 1977 tape of Lennon singing the song was recorded on a mono cassette, with vocals and piano on the same track. They were impossible to separate, so Lynne had to produce the track with voice and piano together, but commented that it was good for the integrity of the project, as Lennon was not only singing occasional lines, but also playing on the song.  Although Lennon had died in 1980, Starr said that the three remaining Beatles agreed they would pretend that Lennon had "gone for lunch", or had gone for a "cup of tea". The remaining Beatles recorded a track around Lennon's basic song idea, but which had gaps they had to fill in musically. Some chords were changed, and the arrangement was expanded to include breaks for McCartney and Harrison to sing extra lines. Harrison played slide guitar in the solo.  The Beatles' overdubs and production were recorded between February and March 1994 in Sussex, England, at McCartney's home studio. It ends with a slight coda including a strummed ukulele by Harrison (an instrument he was known to have played often) and the voice of John Lennon played backwards. The message, when played in reverse, is "Turned out nice again", which was the catchphrase of George Formby. The final result sounds like "made by John Lennon", which, according to McCartney, was unintentional and was only discovered after the surviving Beatles reviewed the final mix. When Starr heard McCartney and Harrison singing the harmonies, and later the finished song, he said that it sounded just like them [the Beatles]. He explained his comment by saying that he looked at the project as "an outsider". Lynne fully expected the finished track to sound like the Beatles, as that was his premise for the project, but Harrison added: "It's gonna sound like them if it is them... It sounds like them now."  McCartney, Harrison and Starr all agreed that the recording process was more pleasurable than when they later recorded "Real Love" (the second song chosen for release); as it was almost finished, they had very little input, and felt like sidemen for Lennon.
Question: What were their roles in the song?. Whats the answer?
chosen to engineer the new tracks.