[peterska2]

If this is both a public joinable usergroup and a secondary usergroup, then there is a little cheat that you can use to mass prune members.

First, in the usergroup manager add yourself as a usergroup leader for that group.

Then go to your usercp > group memberships and click on list members.

Check all the users you want to prune, and then click remove from group.

You can the remove yourself from being the usergroup leader again.