For a room based multiplayer game, I am searching for the right backend service. This is the functionality I need: a player can create a new room where 1 - 5 other players can join, after that the game is started and no other players can join that room anymore. It also has to be possible to create a new room for FB friends only. Is this possible with PubNub? Because I can't see directly how to do this with PubNub channels.
Yes, this is possible. There is nothing in PubNub SDKs/APIs that directly assist or prevent you from doing this. It is just a design pattern that you must implement into you app's biz logic. Here are some very high level pointers.
There is some details that you need to work through but this should help you get started.