Only one player may be in Underspun at a time. Upon joining Underspun the player will become the Lumino Spunner and all of the other players will vanish. The only things you will be able to see are the other inhabitants of Spunland. You will not be able to use the Shop or The Bank of Spun when in Underspun. Note all of the other players will be able to see you.
So what else can you do in Underspun? Well, you can create rabbits by illuminating the Spun Grass. Simply walk within range and follow the prompt. The HUD will show you how many rabbits can currently be created. There can be up to 10 Underspun rabbits active at any one time. Although you cannot see the other players, they are actually still there – and they can catch the rabbits. As rabbits are caught active slots will once again become available.