Preventing user creation, but allowing edit
You might want to use the Administer Users By Role module. The 7.0-2.x version depends on the Chain Menu Access API.

After both modules have been installed and enabled, you will be able to grant user edit but not user add permissions to certain roles within the /admin/people/permissions form.

Nice side-effect: The user editors won't be able to alter the site admin's profile, if you don't allow it, as it could cause serious problems.