Click the blue "join" button next to any game to join it. Then make sure to be online and signed into your Xbox/Playstation approximately 10 minutes before start time, so the game creator can send you an invite.
If you don't get an invite, send them a message through XBL/PSN asking for an invite. You can also try chatting with them right on the game lobby page on the100.io. If no one invites you then start inviting the other players on your own.