In the City View, tap on a 'Barracks', then in the pop-up menu, select 'Summon'. 

Tap on the unit you want to summon. Press and hold if you want to summon more than one and a selection bar will show up. Drag the nod right or left to increase or decrease the amount of units you want to summon.  

Note that after you finished the selection, you must tap on 'Summon' to issue the command, and then a sorcerer will march to the Barracks and start summoning.