Think this goes without saying but this will probably be a public group. However this is the condition it is under:
'Public groups can be joined by anyone without the need for invites. There is no limit to how many people can be members of a public group but there are also no restrictions on who can join. Gameplay statistics are not available for public groups or invite-only groups with more than 150 members. Public groups are recommended for game leagues and groups with wide-reaching appeal that anyone can join.'
Downside is anyone can join and you may get a few trolls. But then again, trolls are unavoidable.