hi, I found a solution after searching for an hour or so:
the army tabs are just not showing at the bottom, but they are there!, similar to other screens where you manage troops. the small dot in the bottom middle is the revert changes button, a tiny bit to the left and up of it is cancel and, you guessed it, a tiny bit to the right is the invisible "done" button.
I pressed around a bit until I received the message (name of my companion leading an army) is enlisting in (my characters name)'s army