Well that would be good I am still amazed at how R* were able to make an open world game run at a fairly steady FPS and still look that good on the PS3 and 360, even if it is running at 720p.
It's automatic unless you "make" your own server, then you can just kick people you don't like. Or you can make a private server and invite people. If you don't want to be disturbed, there is also a "solo" option for Online,where no one can join at all. You still need to be connected to the internet though.