Just press the green flag and the project should do everything else for you. Wait till the variable "current_player" is 1 or 2. Then use WASD keys to move around. Spectators can watch the ones moving around and will automatically join when someone leaves.
This is just a basic design that I've created that automatically assigns Scratchers and automatically disconnects people that have left. All it really uses is a timeout kind of function. I don't know how much this counts as an engine, as all it does is deal with connections. My only worry is that not everyones' time-in-seconds sensor is the same (so mine may be 15 seconds while your's is 30 seconds. So two people may end up playing as the same character. While increasing the timeout would help prevent it, it means a longer time for people to switch out (which may be worth the sacrifice). Anyway, enjoy the project. Remix it, etc.
