Agreed - but that's not "Facebook blocking those who aren't allowed access", that's whoever created a page or group setting their own rules as to openness or otherwise, the precise criteria for applying to join, whether moderators are involved or not. Sure, Facebook provides the toolbox that individuals can use, but FB don't decide who can or cannot join a group. In the case of coins, the group's owners / moderators would have to tread a fine line between wanting new blood (especially the young) and protecting existing members from would-be thieves and ne'er-do-wells.