So you're saying it's going to be much more effective to give some of your forum members a special password that they can potentially leak out and give to other forum members?
What's the criteria for getting access to this forum?
You must have some way of disseminating a password to them so there must be some way of identifying them. Is it post count, trophy points, registration date, their age?
In my mind giving a password out to individuals is no different to giving some of them a special user group.
If you're more clear on exactly what you want to achieve and why we may be able to advise a way to automate it.