Let me assume that each member selects a single category assigned to them self. You can then have tags owned by the member as well. In the presentation for that ownership (the drop list), you can set a specific query:
Tag.Category=Member.Category
I'm not sure how you would set multiple tags, which I'm sure you desire.
Alternatively, if the member has multiple categories, then I suggest that members own the categories and categories own the tags. It may not be elegant, but from there you would have a standard drill-down. Open each category and view/add/edit the associated tags.
