Maybe I should have clarified: I am only talking about scheduling texts...
This illustrates the issue perfectly ;-)
6 hours and 12 posts later, you realize your initial post was not clear...
Scheduling is -always- an issue. n-way communications networks among multiple players in multiple projects are nightmares.
Some kind of shared calendar resource would seem the best way; I've never seen it used successfully, and reasons (ahem - excuses - ahem) are legion.
As you are the gatekeeper of a scarce and shared resource, you should be in a position to dictate some boundaries.
Always state a default course of action and a time limit on dissenting inputs. (e.g. "If I don't hear back from everyone by XX:XX your slot is gone.")
This time limit must be reasonable, given the many circumstances detailed in this thread...
But after the 3rd or 4th rehearsal gets cancelled because someone blew off a response, some kind of behavioral use change will occur.
Why does it bother you? Because you believe people should think and act the way you want them to, and they don't and won't.
It's personal for you, and it doesn't have to be. It's business. People will make or avoid making decisions, which will have consequences. Some of those people will then expect -you- to "be flexible" and do it their way even though they are the source of the initial conflict.
You've evidently taken on the scheduling and hosting tasks yourself. Let it be someone else's responsibility. Or else it's your rules.
Let them try that act with a commercial studio and see how well it plays out...