How do I summon a unit from a town to a hero?

You need the "Summon Troops" spell. If I'm not mistaken, it's automatically granted at the third level of the Mage Guild (except in cases where the spell is prohibited on the map). A 10th-level hero can learn this spell from the guild. To use it: in the adventure map, go to the spellbook and... apply it. 75% of the turn will be consumed, so keep that in mind. If you have less than 75% of your turn remaining, the spell won't work (the icon will turn gray).