What about using an override template for editing users from the frontend? You could then remove the inclusion of "edit_placement.tpl" etc. from it, so that users can only edit their details but not the group.
But I guess this isn't exactly what you want to do?
Yep, you're right. That's not what I reaaly want. I would prefer a solloution without messing with templates of the admin site. Is there any other way?
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Nothing is impossible. Not if you can imagine it!
Can you assign your limited role policies to the user group that these users are in? For instance, you could delete the permissions that give them access to the admin group, and create a new policy which allows more limited access. For instance, they could only read from the users/editors group instead of the entire users subtree. Optionally, you could make this user group a different section, and then limit that group only to see users in the new section.
There are probably several ways to do this using role policies and/or sections.