In pvp at least you purchase them from the spawn menu at the bottom, you order them to a position, then you click the green arrow to deploy them. That makes it so you can then enter your infantry into the position as though they are entering a vehicle from base game. You use the gear to delete the formation.

As far as other formation mechanics I would like to know more of how they work as well, especially if you can/will be able to move men in formation. Would be very cool.