Okay I found how to do it.
When you create a player room, you have to change the very first setting.
- The first choice is the default one (it's white).
- Second choice is what you want. Everyone in the room can plya at the same time. If you have this optiion selected, in the room there will be 8 slots.
- Third choice is training mode. It's basically training mode but online with someone else.
- Fourth choice I don't know what it does.