You can select 3 squads to follow you to your mission, out of a total of 8 teams: Commandos, Scoundrels, Frognauts, Mechas, Fellhood, Ascendii, Cathros, and Daikaijus. 

Selecting your squad

From the Map, tap the Squad icon on the bottom center. 

You'll find your current Squads under SELECTED on the right column. 

If you wish to change them, tap the desired Squad and then SELECT.

Tap which Squad you wish to replace to confirm.