Yeah, this is a common issue on Steam even with everything set to public. Sometimes Steam’s privacy settings can still block invites unless the person is on your friends list, even if Destiny settings are wide open. Double-check your Steam settings under: Steam > Settings > Friends & Chat > Privacy Settings, and make sure “Who can send you friend requests” and “Who can join your game” are both set to Everyone. Also, in Destiny’s in-game settings under Roster, confirm fireteam privacy is set to Public or Invite Only — not Closed. If everything’s already set right, a workaround is to use Bungie’s Fireteam Finder on the app or website — people can invite and join you from there without needing to be friends.