Here is my question. I will use the first campaign scenario Aftermath for the question. I know I can use my threat for the units the you can deploy during setup, like the stormtroopers and the probe droid after they have been defeated. What about the reserve group. Can I use threat to deploy them at all? After they have been defeated? Thanks for the help.

Yes you can.

No and yes. You cannot deploy them with Threat. The campaign guide will tell you when you can deploy them. Once they have been deployed you can use threat to reinforce them or redeploy them when they have been defeted. Open Group can be deployed using threat in the status phase.