Reserve them. Can reserve an entire one if you pay for it.
They are "first come, first served". And you will want the majority of your group to arrive together, as you are unable to "reserve" the entire shelter if you don't have people present to use it all and the park is packed. (You might have to share, rarely. When I go with my wife, we don't crash any parties, but we have shared a shelter with other groups since it's just the two of us)
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