Setting up your units can be done via the Formation icon. You can set up to 10 units with 5 members each. You might want to set some units for specific unit skills.
You start off with an empty panel. Click on each panel to set your members. The first unit and its leader will be the support set-up for your friends as well.
Once formed, save the current formation by tapping on the yellow button.
Using the "Recommended Formation" function (can be filtered) will give you your 5 most powerful cards.