myBalsamiq does not have a read-only permission setting for project members. When you add project members to private projects they have permission to edit Mockups as well as comment and create new Mockups.
If you are worried about project members making unwanted edits, remember that all changes are tracked in the Project History and can be reverted at any time.
The only read-only options we provide right now are these:
You can make the project public and select website or blog access settings.
Use the Prototype view to give access to view the Mockups only. It uses a long string which isn’t guessable, and bypasses the login/password mechanism.