1) Like News/Activism, General Interest, VetsCoR, Campaign2002, Religion: the forum is wide open, anybody can read, anybody can post.
2) Like the FAB and Moderator forums: restricted access, only members can read, and members are added by the Forum Administator.
3) Combination. Anybody can read the forum, but membership is required to post. Members are gained by joining and being approved, or having a Forum Administrator add you.
And while we're on the topic, each Forum has Forum Administrators, Forum Moderators, and then just Forum Members. Admins are responsible for the membership list, managing the moderators, adding groups and topics, forum homepage, etc. Moderators are responsible for keeping riff-raff out, handling flame wars, etc. And of course members are there to participate, to be the forum.
(This means that each forum has its own ban list.)