not Always a Beatle, aka
Guest Musicians
on Beatle recordings
George Martin
(their producer on every album except Let It Be 
)
was the 2nd guest musician, but the 1st notable.
On early recordings, most of the tracks were just a live performance recorded, no overdubs to speak of, so when a piano part can be heard it's probably
George Martin
(in my opinion the only real candidate for the title of 5th Beatle, some call him the 6th?)
Amid his many piano contributions
Sir George
is said to be playing the
Celeste
(hammers strike metal bars rather than strings as on a piano, note smaller size and fewer keys as well)
on
Baby It's You,from their 1st LP 
On
Rubber Soul
, George Martin is given credit on the album sleeve for playing the Harmonium
It's a full sized console version, foot powered positive pressure reed organ,
on The Wordon In My Life that's George on piano (double playback speed, to give it that Baroque feel) not a harpsichord as it may sound.
In the same project
Mal Evans
(long time Beatles insider, former bouncer at the Cavern, drove the van from gig to gig in the early days)
is playing the Hammond inYou Won't See Me
(& we didn't!)
Mal
also participated in SPLHCB
by way of the Harmonica(Mr. Kite) & the alarm clock
that rings during the A Day In The Life sequence
In some stripped down versions (as in the Anthology tracks) you can hear Mal counting the time in between that is later filled with the orchestral crescendo.
(one of several on final piano chord too)
Tambourine during MMT 
& the White Album
as well as trumpet, in the latter project
He's also the bass drum player in
Yellow Submarine
Mal is playing the anvil in
Maxwell’s Silver Hammer, in a sequence of the filmLet It B
Ringo plays it on the record.It's said, that in addition to numerous uncredited writing contributions from Mal, he was the inspiration for the fictitious Sgt. Pepper. When eating together it was Mals way to ask for the salt & pepper, by saying "would you pass me the Sgt. Pepper please?"
Nicky Hopkins
(famous British session player, the Who, the Stones, Led Zepplin)
is playing the electric piano
Fender Rhodes suitcase 73, some say it was a silver sparkle model!
during Revolution 
on the single version (Hey Jude was the A side).
Billy Preston 
(D-152 & RT-3)
& Rhodes Piano
on the White Album & Get Back sessions. During the roof top concert as seen in the film Let It Be , he looks to be having the time of his life!
most consider Billy to be the 5th Beatle
Various keyboard contributions can be heard on
Abbey Road
, as well from Mr. Preston.
_________________________________________
Another White Album veteran was
Eric Clapton
playing the lead guitar in
While My Guitar Gently Weeps
Due in part to John & Paul's lack of interest, George insisted that his friend Eric play on the track. When listening to the playback, Eric didn't think it was Beatle-y enough so he suggested that they add ADT* so that it would sound a little wobbly! It's rumored that he played on other tracks too, but I can't confirm or deny that.
* see There's A First Time For Everything
for more on ADTSo who was the first guest Musician on a Beatles recording?....
Andy White
a session drummer brought in by producer George Martin, after deciding that Pete Best...wasn't!He told Brian
" I don't care what you do with him on stage, but for the recording, I'm bringing in a professional drummer."It seems that great minds think alike and the Beatles were fed up with Pete
He wasn't showing up for gigs, and he wasn't really a Beatle anyway! ( that is to say he didn't fit in, he wasn't as out going, was sort of sullen and to himself & that ain't no Beatle hair cut! ) So Brian was given the task of giving Pete the "axe".
Ringo had been a long time friend, the professional drummer with local band
Rory Storm and the Hurricanes. He would sit in with the boys (often when Pete didn't show up)You know somehow, when things click!
Ringo was all set for the studio gig-- the new drummer and all...but nobody told George Martin of the change ...so in walks Ringo, expecting to play and there is
Andy White! From the producer's point of view, "burned once...not twice, we'll use the pro." Ringo was hurt, and for years he would needle George Martin by saying, "You wouldn't let me play would ya." This little incident healed over years ago, but is quite amusing now.on to Part 2
