Hi Chris,
The maximum # of group members is controllable by our support team.
Just send a request to our support team, indicating the name and appId of the app, and what you'd like the max to be and why. (A short description of your use cases would be helpful - since we just need to review to see if we think there would be any performance gotchas).
We'll do a quick review and hopefully approve!
Cheers!
Paul.