Oh, not quite picking it up, I've been playing bass for 30 years. You're right, though - you're remembering a post where I asked about joining a band as a bass player, because I have been playing guitar for 35 years

and am more used to collaborating as a guitarist. (I also had no idea how to just pick up and join a band although you were quite correct that it isn't very difficult, particularly in Los Angeles!) I can also play keyboards but there will be someone better than me for that role for sure. I'm qualified for backing vocals.
The bandhub model is fascinating. I'm still not entirely clear on how the people get together; some of the covers I listened to (I didn't check out any original music) were apparently the product of sort of a Wikipedia-type situation with people pitching in with parts that they knew/could play in an open-source-type environment, while other collaborations seemed to have been planned in advance.