If you create an army and then select the army you should see build options like you do in a factory. You then click on them to queue up those reinforcements. You can see what you have queued up by looking for the little +4 (if you queued 4) on the army information panel just to the right of the minimap.

It will then queue those units in factories and they will make their way to you upon emerging from the factory. The substrate also have an ability where the units will teleport from the factory to the army directly. Do do that click on the orbital ability then on the army and then start queuing your units.

The PHC have a similar thing but they use a unit to facilitate the teleporting instead of the orbital ability.

All hail warp prism.

Mrrgh... wrong game.

Sorry.