This is a bit difficult to work out, because having too many peas in the pot might make things frustrating. I was in a similar situation with just three people sharing a recording space and there were times where we'd walk in and say "hey, what are you doing here? I need to work on this project" and then there's a hissy fit...

...I was also in a similar situation with one of my professors who opened up a nice studio in Murfreesboro and I offered my gear and time towards building/wiring the studio and he kept brushing me off and there was so much scheduling conflict and lack of communication that I painted a studio, wired up an HD rig, and donate my time and some money and never got a single session, so I'm a little weary.

If you decide to do a buy in, it should be selective. Everything boils down to money, so if a band said they'd give $500 and some time building for a week in the studio that would be worth considering, but it would be easy to get ahead of ourselves and go "wow, this thing has been done for a month and not many of us have used for our own projects because we're still paying back out debts to people." Then we'd have to decide who's going to engineer those sessions, are engineers going to be available (I think we all have day jobs 5 days a week) and are they going to get paid from the $500.