The Diamond Stage feature allows up to four players DJ for a large audience. There is only one Diamond Stage performance at a time, so you’ll have to buy a slot using the Diamonds currency. To do that, select Diamond Stage from the Main Menu and then select the Buy a Slot button.
The price of a slot goes down as the set’s start time approaches, so you can try waiting to get a better deal – but the longer you wait, the better the chance that all of the slots will be snapped up by other players! (Don’t worry: there is a new Diamond Stage set every half hour, so if you miss your chance you won’t have to wait long for another shot.)