i.e. We have an over 18 forum, if they're over 18 (see related thread) and they've ticked the box in the preferences for gaining access to the forum (i.e. they have to opt-in to it).
This would also be important for us in the future with our expansion plans as well, it would make life a lot easier if we could use the usergroup promotion system to promote a user who ticked a custom profile field option.
i.e. we have a Land Rover forum and a Aston Martin forum (for example), however they need to be opted into by the users, so as a custom profile field we'd setup an option for 'Extra manufacturers' and have 'Land Rover' and 'Aston Martin' as options.
Access to these forums would be granted by usergroup permissions, so the usergroup promotion would need to in effect say, "if the user has clicked the Land Rover option in the 'Extra Manufacturers' profile field, then add them to the 'Land Rover owners' user group.
Hope that makes sense!