When I initially created the site I inherited permission. Later I broke the permissions and created some new groups. The problem is that my users with Full Control can't add more users. Under the "Group Settings" I have the Owner set up to edit membership, but it's still not working. Owner's get an error saying "you do not have permission to add users to the selected SharePoint Group."In testing, I created a site and created unique permissions right from the start and my Owners have no problems adding users. I get now that I have a new best practice of always creating unique permissions. But what do I do with all my sites that need to be fixed? Help?!Juliesan

I know this is a really old post but I had the same issue and wanted to share the solution in case somebody else stumbles onto this post. If you create an "Owners" type group you have to set the owner of the group to be the group itself so all members of the group can add/remove users. If you break inheritance and create a new group then by default the user that created the group is set as the owner and as such only that user can edit the membership of the group.