I'm not sure who he is (it's certainly not Luther) but I've always felt sorry for the second backing vocalist. He's clearly singing, but the audio engineer must have left his mic down throughout the song. The tracks in mono, so it's not like he got lost in the opposite channel (say, for example, if we're only hearing the left channel of a two-channel stereo mix.)
Too bad, it's such a lively performance. Bowie's a little hoarse, Geoffrey's a little high in the mix, but well sung none the less.
It's amazing how much charisma Bowie had in those days. There was nobody like him, he was a different creature. This lasted from Ziggy Stardust to the Lodger era.