It looks like 8 per event in the running events (2 per event per team).
You can't still be in college to participate. Have to be a U.S. citizen.
Here are the top 8 runners in each event for 2016 (counting indoor times).
You would think as long as they have registered, they would be considered first based on the fact that they are running well right now.
800
Berian 1:44.20
Soloman 1:45.47
Windle 1:45.65
Sowinski 1:45.80
B. Johnson 1:46.12
Murphy 1:46.13 (indoor)
Gilreath 1:46.70
Abda 1:46.75
1500
Centrowitz 3:35.91 (indoor)
Torrence 3:36.05 (Peru)
Murphy 3:36.23
Alexander 3:36.26
Gregorek 3:37.36
Andrews 3:37.52
Blankenship 3:37.80
Callahan 3:37.87 (Belgium)
(True has run 3:36.05 but more than likely would opt for 4 mi. road race)
Steeple
Jager 8:15.26
Kebenei 8:18.52
Huling 8:18.58
Leslie 8:19.12
Cowart 8:23.28
Bayer 8:24.37
Mahoney 8:25.44
Bor 8:27.01
4 Mile Road Race (5K times)
Mead 13:04.17
True 13:12.67
Kipchirchir 13:18.52
Torrence 13:19.42
Rupp 13:20.69
Chelimo 13:21.61
Heath 13:23.06
Jenkins 13:24.67