How to add the "group register" field to the user registration page (special case)

Here is what I would like to do. I have OG's created and would like the OG Manager to be able to add new users while at the same time subscribing them to the OG. I have successfully added the "Group registration" field to the User registration form, but bizarrely it only shows up either for the super-admin, or when registering a new User, not for the OG Manager. I guess there must be a problem with permissions but I have done everything I can think of without success.
The Group manager should be automatically accorded "administrator member" status on creating a group, and the "administrator member" is allowed to add users without approval - and indeed he can add users once they have been created. But no way can I get the option to appear on the "Add User" form
Help desperately wanted!