My flake rate is close to zero, mostly because people personally told me they’ll join.
It doesn’t hurt to get the group chat hyped up on the day of the event. The activity is enough to get people excited. I also pin the time and location so people find it easily.
Besides that, just chill. It doesn’t have to be perfect. Once a few good people are there, the thing mostly runs itself. Try to relax and enjoy your own party.