As mentioned here http://www.rockrms.com/Rock/Ask/Troubleshooting/Questions/809
- perhaps the GroupFinder block should not show groups that are at their max member (active) count
- the externally facing GroupRegistration block is not checking the group's MaxCount (active members) before adding the new person.
(The GroupDetail block does check the MaxCount).
As mentioned here http://www.rockrms.com/Rock/Ask/Troubleshooting/Questions/809
(The GroupDetail block does check the MaxCount).