SELECT a.user_id, a.info_display_name
FROM fbb_user_info a, fbb_user_groups b
WHERE a.user_id = b.user_id AND b.group_id <> '12' AND a.info_avatar IS NOT NULL order by user_id
You will have to substitute your group id number for your subscription group where my group number is 12.
This should work but if they are in the another group as well as the subscription group, they will show up in the list. I'm not sure how you have your forums set up, whether everyone is in just one group or can be in multiple groups. It might need tweaking but it is a good start.