Buying a construction site is cheaper. For only 10k you can choose between depot, trapper camp, outpost and scrapheap. On top of that you can choose your own location.
Make sure all of you are close enough to the base seller.
Should be enough, but to be sure just simply stay one hex to each other and 1-3 hexes from the base seller.
try now, doesn`t work
is one of your friends in a faction already?
no
But maybe in NPC driven faction? Like BOS, Vault City or so?