I think that the only user types with the ability to post public entries are Moderators and Administrators; it'd be nice to expand this to a new user group as well...
not sure how that'd work with a new user group, theres a couple of ways of doing it:
1. New users are added to the new group by default and you can remove their ability to post in the calendar by removing them from the group.
2. You must add new users to the usergroup to enable their ability to post.
Myself I'd pull for #2 should this ability ever be added.
