Press the green flag ONCE and then wait a minute. It may take a while. If you are not connected, make sure you are not a New Scratcher and that your internet connection is stable. Press the debug button. If the numbers are changing, it means that someone else is connected. Otherwise, wait for a slot to open.
THIS TEST CAN ONLY TAKE TWO PLAYERS Used the Scratch Wiki for help on a lot of the multiplayer aspect.