Quote Originally Posted by Cheexsta
You just need to press G, no Ctrl needed.

Personally, though, I tend to avoid groups at all times unless I really need to keep my whole army in formation easily. Even then, I tend to remove groups before I even make contact with the enemy - there are just too many bugs associated with groups, particularly with cavalry and missile troops. Cavalry that have been grouped together will charge the nearest enemy rather than the unit you're targetting, and missile troops will do the same with their missiles rather than concentrating on a specific part of the enemy line. It's just plain frustrating.
You can use ctrl + numbers as well, then you get to decide what number the group gets.
I like them mostly because it saves their formation, making it quicker to order them around in good order when maneuvering before the lines meet. Probably even more useful in MP where there is just no time for micromanaging every unit.