I added a new feature to current git version which allows "shift" incombination with group recalls.
So if you have bound some unit groups to the numbers, you can recall those while holding the shift key. Now the groups are added to the current selection.
Please test this as this changed something with the key handling ( does this work on windows ? )
Additionaly I added the possibility to add units to an existing group. To do so you select a unit ( or some units ) and then you press ctrl+shift+groupnumber. So Ctrl+shift+groupnumber adds the current selection to this group. I am not sure if this is too complicated and if this is really needed with the new feature A) above. Please try it and tell me your opinion.