Umm, since nobody else really said anything helpful...
if you have a mixed group of muskets/skirms/hussars, if you double click on any one musket it will select all of muskets around that particular area. This works for every unit.
You can't make groups of soldiers over 50. 50 units is the max a group can hold.
If you are trying to group with hotkeys, ctrl + number makes a group. Shift + number adds to a group. Groups are still capped at 50, a unit can only be part of 1 group you access them by pressing the number on the keyboard. Hope this helps.
This works for buildings as well. I like putting 4-5 rax grouped together on 9 and quickly selecting them to make a lot of infantry...