Only logged in members of PC can read any social group. Browsers, who are not logged in, cannot even see the social groups. They would not even know they exist.

Social groups can be public, or invite-only.

Invite-only groups can decide that only members of that group can read messages on that social group.

Social groups may have social group forums, and if the group is invite-only of either kind, only members of that social group can read messages on their social group forum.

I think this is correct.

For example, I created a forum which is now invite-only. All logged-in PC members can read messages on the social group, but they cannot see messages on the social group forum which is linked to it.