Scirra provides a signalling SERVICE. This is used to connect peers to each other. As the multiplayer plugin utilizes peer to peer networking, the first peer to connect to a game becomes the host, and that is your "server".
To control who connects to your game, you will want to use a unique Game Instance name. Then only people running the same game will be able to join.
You should read, follow, recreate, and understand all of the multiplayer tutorials before trying to make a multiplayer game to avoid running into significant difficulties down the road. Good luck.