User opt in or out of a "flaming" forum

[The Piper]

Hello there,

My users are requesting that I open up an unmoderated forum, where they could discuss everything they want (including topics such as religion, politics, etc., which can generate big arguments). My mod's conditions to it, though, is that users would not, by default, have access to such a forum. They would have to agree to some special terms and then they could opt in, that is, they would then have access to the flaming forum. At at time, they could opt out of it.

Users with no access to it would not even know that the forum existed.

I have no clue of how to do this. I thought of having a special usergroup which would have access to this forum, but I woudn't want to have to manually add each user to this usergroup.

Any ideas on how to achieve this, please?

Thanks a lot,
Paulo