Wasn’t sure how the categories were being associated with the users so I checked the tutorial you linked to and found this:
$user_meta = get_user_meta( $user_id, USER_CATEGORY_META_KEY, true );
It’d be whatever your code’s version of that line is… so whatever you’re saving the categories as in terms of user_meta, you’d call that. But change the true to false so you get all of them instead of just one. From there you should be able to parse all of it however you want.