Hey, sorry if im wrong because i just had a really quick look at the posts, But adding a password to the Invite only forum would be better ? Then you can PM the members the Password. The usergroup idea is good, but Depends on the Users preference.
- Forum Password ? if you wish to password protect this forum, specify the password here. Administrators, super moderators, and moderators of the forum in question will not be prompted for a password. Passwords are not automatically applied to child forums; use Apply This Password to Child Forums to emulate this function.
- Apply This Password to Child Forums ? if you have changed the password for this forum, setting this to yes will apply the specified password to all child forums. This can be used to emulate inheritance of passwords.
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Vbulletin Online Manual