If he was paid for it, he was technically a session musician. The fact that he was uncredited shows he was never a band member.
George Martin did a lot more than just play a few piano bits - he had a lot of creative input. Here's what "The Beatles" by Spitz has to say about it:
"Two weeks later, on his way to the Studio from Sussex, scheduled to give the song another shot, he was explaining to Eric Clapton how something radical was needed to light fire under the Beatles. "We were in George's car, driving in London," Clapton remembered, "and he said, 'Do you want to come and play on this record?'" It was an astonishing invitation. The Beatles had used plenty of Session musicians on other albums, but no one capable of upstaging them, certainly never rock 'n roll virtuoso on the level of Eric Clapton. Clapton hesitated, unsure of what to do. He knew the other Beatles "wouldn't like it," but George brushed aside his reservations. "It's nothing to do with them," he insisted. "It's my song, and I'd like you to play on it."
Before anyone had chance to object, Clapton was already in Studio Two, strapping on his Les Paul guitar and listening to the rhythm track mixed down from their work on the sixteenth. The song was pretty much there, creating an effortless, affecting groove, but it lacked dramatic device to liberate the emotional tension that is never far from George's caged expression. Clapton's poignant guitar riff provided everything it needed. The way it weeps and moans, held in check by Eric's incisive phrasing, creates the longing that gives the song its emotional center. George's vocal couldn't have been more enchanting as he squeezes the mournful lyric of all its desperation, until by the end, he seems to be just barely hanging on, just riding atop the surging guitar as it works to strangle his overlapping cries."
George Martin did a lot more than just play a few piano bits - he had a lot of creative input. Here's what "The Beatles" by Spitz has to say about it:
"Two weeks later, on his way to the Studio from Sussex, scheduled to give the song another shot, he was explaining to Eric Clapton how something radical was needed to light fire under the Beatles. "We were in George's car, driving in London," Clapton remembered, "and he said, 'Do you want to come and play on this record?'" It was an astonishing invitation. The Beatles had used plenty of Session musicians on other albums, but no one capable of upstaging them, certainly never rock 'n roll virtuoso on the level of Eric Clapton. Clapton hesitated, unsure of what to do. He knew the other Beatles "wouldn't like it," but George brushed aside his reservations. "It's nothing to do with them," he insisted. "It's my song, and I'd like you to play on it." Before anyone had chance to object, Clapton was already in Studio Two, strapping on his Les Paul guitar and listening to the rhythm track mixed down from their work on the sixteenth. The song was pretty much there, creating an effortless, affecting groove, but it lacked dramatic device to liberate the emotional tension that is never far from George's caged expression. Clapton's poignant guitar riff provided everything it needed. The way it weeps and moans, held in check by Eric's incisive phrasing, creates the longing that gives the song its emotional center. George's vocal couldn't have been more enchanting as he squeezes the mournful lyric of all its desperation, until by the end, he seems to be just barely hanging on, just riding atop the surging guitar as it works to strangle his overlapping cries."