C++ finding n points as close as possible to given xy

Question

In RTS games, when you move some units, they find path and go to the places that are the closest to the selected place. I dont know how to select those places, I mean the target points for each unit.

For example, when I send 9 troops, I want them to have TARGETS like this:

. - empty, 
T - targets for units, 
O - the place that I've choosen to move them, target for unit too
.....
.TTT.
.TOT.
.TTT.
.....

Pathfinding algorithm is ready, just I need to generate the list (or vector) of target points, one for each unit. I dont want the complete code, but just some advices and ideas... Well I have to mind that not all places are walkable...

Thanx for any replies and sorry for my bad english...