When a group of the same unit are selected (e.g. 12 villagers) you can modify that selection with:
LEFT ARROW KEY: divide the group selection by half (only 6 villagers are selected now). Hitting the left arrow key again divides the selection by half again (only 3 villagers are selected now).
DOWN ARROW KEY: minus the group selection by one.
RIGHT ARROW KEY: Multiply the group selection by 2 (only works if the selection was reduced previously).
UP ARROW KEY: Add the group selection by 1 (only works if the selection was reduced previously).
END ARROW KEY: Inverse the selection (if 9 of the 12 villagers were selected the inverse would be 3).