How does one maintain order among volunteers?

I’m directing a play. It’s the first time I’ve directed, and I’m enjoying myself.

I have one area that is not going well. We aren’t starting on time.

How do I discipline a bunch of people who are volunteering their time? Everybody has busy schedules. Am I herding cats? (No, that’s what it’s like to direct a bunch of drag queens. I know. I’ve seen that as a drag queen myself.)

Any tips would be greatly appreciated.