Expand the possibilities of setting the user's permission of the message boards

Currently it is not possible to share one specific board with external guests with full edit permission, and at the same time share it read-only to all users in the organization with the exception of a few users who can edit.

The reason is that there are only 2 levels of users (ordinary user and owner), while the guest cannot be set as owner.
Perhaps it would be helpful to have the option for guests to set their own permission to edit a given board.