I want to let certain members lead usergroups but currently if I put them in charge they can just add as many members as they like to their group without the aforementioned members doing anything. How can I set it so that they can only approve people who join the group by choice?

Or do I have to be the group moderator in every case to prevent this from happening?

I don't actually think that's possible. You'd at least order them to stop doing that unless they apply or something.

That's right, there's nothing that can be done to stop them from adding new members to the group. All you could do is to ask them to not to do it.

yeah and if they continue to do it then just say they will no longer be a moderator on your forum.

You can always check you logs to see who is doing what, but i am sure if you tell your staff you only want admins to place members in groups, that thy will respect it.. or set up a staff board where you moderators can post and request a member to be placed in the group.