[Steve]

I want non-logged in users to be able to read the forum but not post to it.

I want them to become members to post to the forum. I am using buddypress and paid membership pro.

Does it default to this, or do I have to set this up? How?

Thanks.

[Robkk]

It should default to this. I think anonymous posting is disabled by default in Settings > Forums.

If you have some hidden or private BuddyPress groups where only members of the group who are logged in can see it, then change the groups to public instead to allow non-logged in users to read your group forums.

Make sure your forums are not private or hidden as well.