If I understand what you want to do, you can't respawn a group of NPC ships.
What you can do is make an identical group spawn when the original one gets destroyed. Just put down a group of the same ships, then in the States tab, set them to go from invisible to visible on component complete. You should see a button appear under that with an exclamation point, this button will open a menu, in which you should select the original NPC group.
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