[s0lidgr0und]

Just use PMs if it's only going to be used to resolve issues with that specific member.

Or you can create a usergroup called Limited Access (or whatever you want to name it) and also create a category that only the Limited Access usergroup can view. When that person has an issue you can grant secondary membership to that person, create a thread to deal with what you're dealing with, and then when you're done you can withdraw the secondary membership to that usergroup.

Seems like a lot to go through, but it's a strange request.