Using a crib when you song a song, that's common enough, and it's even traditional - the Copper Family used to for example. It can be overdone; it should be used as a way of helping you sing a song you already know to sing, only the words are a bit slippery or rusty, not for songs you don't know.

But what seems strange is the idea of a bunch of different people using the same words book. But then, if it makes for an enjoyable session, fair enough. Singing isn't about competing to see who's got the best memory.

Actually it can be when you know a song so that you sing it without making any effort to remember it, just open your mouth and the words come out naturally, (which is by far the best way to sing), that it's handy having the words there in black and white. Those are the very times when you are liable suddenly to find they are missing in your head. If it's a song you are consciously remembering that's much less likely to happen. But the effort of remembering, while successful, tends to get in the way of giving the song its head.