You can make users leaders of the usergroup. That allows them to approve and deny requests. They don't necessarily have to be moderators. In the usergroup settings, set "Public (Joinable) Custom Usergroups" to yes. Anyone can join the group now until you assign a group leader. Back on the usergroup management list page, click the drop-down menu on the right beside the usergroup, select "add usergroup leader". Then you can type in a username and add them as leader. You can add as many leaders as you want. Then, on this page
, people will be able to request to join the group, and the group leaders will be able to approve/reject. I tried this on a different version of vB from PCG's current one, so it's possible these settings may be different - let me know if anything I described isn't available.