According to THE FROG, it's drag and drop when the two characters are near each other.
I don't know about adjacent units, but drag-and-drop has worked for a while for swaps between units in a stack. You just click and hold on the item you want the selected unit to give away and drag to the medallion for the unit you want to receive the item, then drop.
I'd still prefer to see an inventory dialog of some sort, at least as an option. As of the last beta, managing inventory seemed to be a minor mouse skills test, not my favorite thing in a TBS game.
