People joining friends only servers

Same thing happened to me last night. Made a friend's only server and someone not a friend joined.

really i'm going to try that

Happened to me as well, to my dismay we had started the friend only lobby after running away from someone wrecking our public Co-op attempt, only to have the same person join and continue ransacking the next instance too. 

So the friend's thing is a little more broken than thought. For now we are going to temporarily disable it so people don't fall into this trap. In the meantime, please use the Local Only or add a password. Sorry for the inconvenience.

Happened to me and my friends too, played a friends only game and three different people joined in with neither of them being in our friends list. 

Changed Status to Known Issue