From my own experience, when I was visiting Barcelona, I only needed my passport to join they just checked my age and ID, nothing more. I actually found https://cannabisbcn.com/ while figuring things out, and it helped me understand the process before going. No one asked for proof of where I was staying or anything like that, but some clubs did mention invitations or pre-registration, so it really depends on the place.
If I'm a tourist and want to join a cannabis club in Barcelona, is it enough to just bring my passport, or do I need additional documents like proof of where I'm staying or a recommendation from an existing member?