(Based on my playthrough)

Nate:
From now on, we'll be using code names! You can address me...as Eagle One!
Preston! Codename: Been There, Done That!

Preston:
._.

Nate:
MacCready is: Currently Doing That!

MacCready:
*finger guns*

Nate:
Piper is: It Happened Once In A Dream!

Piper:
*fist pumps*

Nate:
Hancock is: If I Had To Pick A Dude!

Hancock:
Alright-

Nate:
Nick is:...Eagle Two!

Nick:
Oh thank god-