Prevent user from adding themselves to administrator group2 users found helpful
What you are really asking, then, is if there is a way to prevent a user from adding themselves to group X, where X is any group, and the answer is, "No". If the user has access to User and Groups, then they can add any user to any group, including Administrators.
In order to change this behavior, you would have to alter the code. You could put a check in the section that lists groups. If the user is not in the Administrators group, it doesn't list the Administrators group. You could even get fancy and only allow a user to add people to the groups to which the user belongs. So, they would be a "group admin". That sounds like that might have some potential. I just might have to look into that. :-)
So... Admin Group is not really dangerous? I was worried about the client adding a user and then adding it to the Admin Group and then destroying the site.