you could do that with vb2 usergroups and premissions
the kidna the same with vb3
with vb2 youd make a usergroup and edit its premissions
with vb3 you could have a usergroup manager
so that could be your clan leader, he would process applications to the user group
you could setup the registered usergroup not able to see any clans forums/subforums
and setup the clan usergroups to be only able to see their own clan forum plus the other forums if you have them by using multiple user groups which is possible in vb3