Group lunch orders for office teams

Your food app lets office teams order lunch together. One person starts a group order from a restaurant and shares a link with the team. Each person adds their own dishes to a shared cart. The organizer then taps Place order and pays for everything.

Everyone in the group should see each dish added, changed or removed within 2 seconds. People can change only their own dishes. Once the organizer taps Place order, nothing can change, and the restaurant gets exactly the cart the organizer saw.

A group has up to 30 people. At the lunch peak, about 4,000 group orders are open at once, with about 15 people in each. A group order stays open for about 20 minutes, and each person makes about 5 changes.

How does a change reach everyone else's screen, and what does Place order check before it charges the organizer?