Q: Can I give some users read-only permission?

To a certain degree, yes. In a forum's administration pages, you can specify that anyone not logged into the forum (whether via the shared password, or using individual accounts) can still get in, but only view the forum, but not make changes. Anyone who does get in via providing the appropriate password will have full read-write access to the forum. You can't say that certain accounts get read access, and certain others get read-write access.

If you would like this kind of fine-grained control, it is available in the Enterprise version of ProjectForum (or the corresponding Campus version of CourseForum). The way people login to the system is a bit different than in the standard version, but it does give you the flexibility to say exactly who gets what type of access (read, read-write or admin) to any forum on the site - group or project.