Well… no, we can’t, because there’s no difference to be sorted out… whatever @codinghorror said then either changed or was misunderstood.
The ‘everyone’ group always has id 0 in the
groups table. If you set a category to be visible to everyone but only writeable for another group, an entry referencing that group id appears in the
category_groups table. As long as this entry exists, guests can see posts in the category, hence guests who are not in the database are included in the group ‘everyone’.
group_users table, which determines group membership, never contains any records with group id set to 0. Membership in the ‘everyone’ group is implicit and applies to every user – regardless of whether that user has authenticated and is represented by a model in the database, or not.
If you want a group that includes all registered and logged in users, that’s the
trust_level_0 group – but I’m not sure right now whether that will also include users who have since been promoted to a higher TL. (Edit, apparently it does.)