Kocmoc made a video guide for it:
https://forums.totalwar.org/vb/showt...ncluding-Video

but the basic way is to highlight each unit you want in the group and then hit CTRL-1 (to make it Group 1, or CTRL2 to make it group 2). you can also highlight them and just hit G and it will group them into a group and give it a number. Then when you want to call up that group you just hit 1, or 2, or whatever.

you can hold down CTRL when clicking on unit cards to add the unit to your selected units without dropping the other ones you have already selected. You can click on one unit card and then hold shift and click on another unit card and it will select all units in between the two clicks.

Watch Kocmocs video and then try it in game.