Game room -- synchronized start NodeJS

This should be a relatively simple task, but would appreciate multiple opinions. What would be the best way to start a game for everyone at the same time (developed for iOS)? Here's the scenario:

1. One person (creator) creates a game room
2. Multiple people join
3. The creator presses "start game", at which point everyone in the "room" gets a countdown (synchronized across all devices in the game room)
4. People play game for XYZ number of seconds
5. Scores are saved and the high score within the room is broadcast to all players in the room

Sidenote:

• Using NodeJS
• Considering Longpolling and Websockets (don't know which, if either of these, would be best for this case)

Would love your advice. Also, I very easily could've missed an important detail, so let me know if I could clarify anything. Thanks!

via amin0