[.918 Bug] How do I join armies?

I have an army with 5 units. I was trading items before a battle and next thing I know, I accidentally attacked with a single unit, so I cancelled.  That unit is now in its own stack out of the rest of the army (but in the same tile).  So I can click on top and switch between the stacks.  How do I join that unit back into the army without moving it to another square first?

(Sorry for double post, posted in the wrong category)