It doesn't say that in the FAQ, you don't even know if you can make it invite only
Just because its not in the FAQ doesn't mean the info doesn't exist. At first I thought your were talking about dedicated servers but I was mistaken.
You can. I know this since the devs have said it in interviews. I know most of what's going down in Brink.
Also the http://www.fragworld.org/frag/brink/the-fragworld-brink-compendium.html
In Brink, players will be able to, at a glance, see where all their friends are and what they’re doing, and jump directly into their games. Once they’re together in a mission, they can opt to stick together so they won’t get split up to different teams or different servers.
Basically, when you’re playing through a campaign, when you’re about to start a new mission, there are two simple choices you can make:
1. do you want to play solo, co-op, or versus?
2. if online, do you want to play with just your friends, or with anyone?
You make that choice and then launch the mission. Depending on what you chose, the game will either:
1. set a match up for you specifically that only your friends can get into, or it will
2. find a public match that gives you everything you need (an open slot on the faction you’re playing, a good ping, etc.),
3. or just let you play completely by yourself with no one else getting in except people you invite.
By the way why ask if you don't trust the answer?