Check in or RSVP in advance

We are starting an adult fitness program. All classes are drop in. It would be nice if there was an easy way to check in or RSVP in advance. There are limited spots, but also, I am paying instructors and for some classes, we have one person show up. It would be nice to know ahead of time so that I might cancel a class if less than 3 have RSVP'd or checked in. I've seen this feature at other studios. I know you can pick drop in dates... but it's a pretty involved process. Can we make this process more simple and straight forward so clients will be able to use it easily ?