How do you fill a garrison with creatures, and how do you combine creatures into a unit?

To populate a garrison with creatures: 1) Create a garrison. 2) Left-click on the garrison (select it), then press the spacebar. 3) In the window that appears, click on the Army tab and choose the creatures. If you want the player to be able to take creatures from the garrison, check the "Can be taken away" box. If you don't want them to be able to take creatures, leave the box unchecked.