Recently I have been playing with some larger groups (10-12 people). It seems like every pending rehearsal causes a flurry of emails (I'm in, can't make it, extraneous suggestions, etc.) I did some research on free group calendaring options. I found one called Teamup that seems to work pretty well without being overly complex. The trick, of course, is getting everyone to use it. With one band we are pretty much at 100% after about 4 months. With another larger group we are at about 80% after two months. The major hold outs are the vocalists, which is a bit frustrating, so I'm going to start nagging them.