Copy the link and open it in a new tab/Incognito Mode.
Go to the first tab, enter the Username and Room Name, then click on Join Chat to join the room.
Go to the second tab (new tab/Incognito Mode), enter the different Username and Same Room Name that is used in the first tab/window, then click on Join Chat to join the room.
here you go you can chat with your friends, and add multiple people to the room, by joining in the same Chat Room.