To have a party at the shelter by the pool do you have to call and reserve it?

Yes

Maddie, darling, what kind of party are we taking about here? Heavy drinking and drinking games? To name a few: Beer pong? Ice luge? With that kind of party, I would say it's always best to call the office.

If your speaking of the one outside the fence it's open to the public, however it can be reserved to ensure another family/ group doesn't. The park has several shelters around the park as well. Hope this helps.