Think I only changed the setting in SMF Admin > Features and Options > General,
Allow guests to browse the forum
While testing I did add a little message on the forum page which is only viewable to guests. You can simply turn it off if not needed, but it might be handy as it tells guest they must be signed in to view the forums.
Thanks. Sure is a lot places to play with in SMF. Aw the ">> Notice: The forums are viewable to members of this site only. <<" is something you added. I thought it might be the standard message but I like it. Thanks!
That message is coded in a php block, and contains a little snippet of php code to stop it annoying everyone including admins, so only geusts shoud see it.
You can edit the message to suite if desired.