IF PARK STAFF ARE ON DUTY: Let them know and they will alert the park guests that the shelter has been reserved for the day by others. IF PARK STAFF ARE NOT ON DUTY: It will be important that you have brought a copy of your shelter reservation posting. You will need to show them the posting with matching date and correct park identified on it. You will need to inform them that your group has paid and reserved the shelter for the day and are planning to set-up for your event. Ask them to please remove their belongings from the shelter area until you are finished.
It's possible someone else unknowingly pitched a tent, not knowing that campsites need to be reserved and paid for in advance. You will need to inform them and show proof of your reservation confirmation.