How do I find out who's in NAP?

NAP is a voluntary agreement. Depending if its for your state or during an event you need to check first with your own alliance leaders or the leaders of the state (top alliances) to see who is covered. If it is a state NAP it is usually amongst the top alliances which you can see by tapping on More > Leaderboard > Alliance might rank. But the agreement can cover anyone that the various alliance leaders have talked to and want to include.

The state NAP can usually be found on the Zenith Tech Hub bulletin, depends on the state. As mentioned, NAP is a voluntary agreement and the rules around it may differ from state to state and how you get accepted into the NAP may differ as well but it generally involves the Wasteland King event but the required participation may also differ state to state.

Check your alliance intro, no good R5 would leave that off of it