I guess Elgg has made things too powerful and flexible. You could grab the addtopic action and override the access level with the owning group's access, and maybe even remove the access input selects from the form. Not too much coding involved there.

@Rho

$user->admin = true;

I wouldn't recommend upgrading to 1.7.1 while you have this problem. Better to correct ir first. Elgg 1.6 used metadata (rather than a user attribute) to indicate that a user is admin. Your best option is to write a plugin that does something like Dhruva is suggesting:

• Register user
• Write a plugin that gets the user using username or guid or whatever
• Set the admin metadata to true

Thanks guys, much appreciated.  I'll start looking into your suggestions this afternoon.