This feature is already here (or something like it).
http://www.njflyfishing.com/vBulletin/groups/
You can create your own groups and grant access by inviting only the members you want to have access or you can make your groups public. You can post to these groups so only the members have access... So if you have a group of people who you share wild trout streams with but don't want everyone on the site to know about it, create a group and invite only who you want.