What would help a lot is for there to be a way for the forum administrator to directly nominate or override the accessible user group(s) and thus categories assigned to the user, while first approving the users - all from the same ‘Approve User’ dialog.
in fact the whole user profile should probably be editable from this dialog - to allow clean-up of identified user errors in custom user fields.
Currently the only way tidy up profile issues on sign up seems to involve a lot of jumping around to other areas in addition to approving the user - with significantly higher risk of error or omission on the part of the administrator as a result.