Social Groups by Waindigo has all the fundamentals that private forum groups need, in a decently designed package. A few additional plugins that extend abilities are also available. They are amazing to say the least.
It's ok. While the finished product is nice, it is a lot of work for an "addon." It also requires other addons, which I don't really like., Plus it really needs a good set of instructions, unless youre a seasoned xenforo'er. It isn't newbe friendly.