The manual is pretty clear... Which part don't you get?
You'll need an ”on room list" event, then you can get all the room information by expressions.
[quote:3qv3qd8b]ListRoomCount
After On room list, the number of rooms in the received list.
ListRoomName(index)
ListRoomPeerCount(index)
ListRoomMaxPeerCount(index)
ListRoomState(index)
After On room list, retrieve information for a room at an index in the received list. The state can be one of "available", "locked" or "full".
You can use a loop for loopindex from 0 to multiplayer.listroomcount to go through each room by index.