BuddyPress.org

This is a basic WordPress question and not a BuddyPress question.

Pages cannot have categories or tags. If you want or need these, then you’ll have to use WP posts instead.

Here’s a link to a WP codex article on pages that will help answer some of your questions.

Now, you could use BuddyPress groups along with Burt Adsit’s BP Content plugin to accomplish your goal.