If I am coming with a group of 15 ppl do I need to make some type of rsvp? This is for Monday evening around 830 pm 11/19/18. How much does it cost?

It is a public park, don't think you need to make reservations unless you are setting up things for a party or something. The park closes at midnight.

I don't think a reservation is needed unless you are hosting a gathering of some sort. It is a public park and there's no cost to access the park.