Any moderator or room owner can grant additional access to any user. Granting room owner privileges or removing them produces a message like the following in chat:
Mad Scientist has removed Mad Scientist from the list of this room's owners.
But granting someone read access or write access doesn't produce such a message, those events are essentially silent.
In the case of private moderator rooms, this is a bit problematic. Adding a non-moderator to a private room should be very rare, and not at all done for the regular moderator rooms that contain private information. But the process of adding a user to a room is entirely silent, and none of the other moderators in chat would notice that a new user was added and can now read the room.
Adding a non-moderator to a private should create a chat message like changing room owners already does.