User controlable groups?

I am looking for a way to create a "custom" group that will have a group leader/admin that is able to invite people to that group.

Reason: I run a minecraft server that has "clans" and I want to be able to give these clans forums. However I don't want someone who is not in the clan to be able to read another clans forum. I can make a user group for each clan and restrict access to only that user group, however I would have to manually add or remove each user to and from the clan groups depending which clan they are in. I would like a way were the clan leaders could invite people and remove people to and from their clan group.

If anyone could either give me a link or make something that would let me do this that would be great :)