When I spending threat, can I bring back a unit from the "Initial Group" or is it only units I selected from the "Open Group"?

Initially, you can only spend threat on optional deployments. However, later in the game, once you've lost people, you can spend threat to bring them back. When the game starts, you should separate your reserved groups from your optional groups. From then on, add any groups that get killed off to the optional groups deck so you can deploy them at any time. Whether they're part of the initial groups or reserved groups you can bring them back once they've been eliminated.

Thanks

Only exception is unique characters. They're one use deployments.