Restricting group creation is something that many, if not most, BuddyPress sites want to have in place right away. Depending on how many options you need, there are a couple of solutions.
The first and the easiest way is to restrict group creation to site admins only. This is actually built into BuddyPress since version 1.5. You can find it under the BuddyPress » Settings menu:
Simply toggle it on or off. If you need more options than restricting group creation to admin only, then you’ll want to check out the BuddyPress Restrict Group Creation plugin by Rich Fuller.