Part of the game will be to try out these combinations and see what summons what. We'll keep you posted as more info becomes available on Summoner. And be sure to check out our Summoner preview and interview with Volition's Mark Allender, if you haven't already. Each has a description and a screenshot or concept art included. Units are obtained in a few different ways.
Heroes can recruit units on their team, but they have to hire neutral units.
In a player's town units are produced in buildings. Additionally a player can upgrade his town structures to enable new or more advanced units. Units can be upgraded by researching a new spell or skill in the Keep. All units are not playable off the bat in the game, but will have to be enabled through different ways including building upgrades. For instance, upgrading the Ivory Tower to Inner Sanctum enables the Sorceress unit.