Party Passes may be used in place of Gems to pay for Hard Rock World Tour Charm Parties. Passes are most often given as gifts but can also be collected through the Treasure Box and games like Sea of Riches.
When any of number of Party Passes are added to your account, either as a gift or a prize, you'll see your Party Pass balance go up to match. Your balance is always displayed at the top of the Party screen.
Note: Hard Rock World Tour always collects Party Passes before it'll collect Gems to pay for a Charm Party. That means if you are given Party Passes while you are in a Charm Party, the game will take those passes to pay for that same party, even if you agreed to pay Gems when you entered.