Why not adapt the principal from starcraft 2? Build some groups (A,B,C,D) (or I,II,III,IV). Each member within the group can be chosen with the keys 1 to 6 (or to 8 ), each group can be chosen with STRG+ 1 to 4.
To help the player orientating even better, the position of each soldier relative to the current selected soldier could be added. In my attachment I did this with (okay, I admit, ugly) red arrows and a red circle for the selected player (I used paint and didn't want to spend much time for it). Other representation like highlighting the respective direction would also be possible, of course.