To set up enrollment for an activity within a members quota, create a general group (under the "Groups" tab) and post a message in your CAS group letting students know about the new group and that they can join so long as there aren't more than X members already. If they are interested, students can click the "Groups" tab, and then click "Show All Groups". Here they can see how many members are in the group and then click "Join Group" if the quota hasn't been reached. If you as an Admin join the group, you will be included in that total, so adjust the quota accordingly.