Adding secondary usergroup to not logged-in user

[Robert Verdam]

Is it possible to add an secondary usergroup to a user who is not logged in?

I want to dynamically add a user to an usergroup depending on the country he's from without him having to log in. So if an english user comes to the forum he can only see forums for his language. I've got this working for users who are logged in, but now i want to do this for users who aren't logged in. (vbulletin->userinfo['userid'] == 0).