[ghaff]

Or you just develop a sense for typical attendance over time.

I run various volunteer activities. Some do require people to sign up for free but we really make it clear to people to please give us notice of cancellation if it really matters to us.

You can charge a small fee. I know events that charge and offer to refund it if you show up. But for more casual evening events, this doesn't really work most of the time. People don't like to feel like they're locking themselves in which they tend to do even with a small fee.