Declare "friend" instead of "Share ownership"
Instead of making those close share ownership for things when built, which kind of forces people to be close to one another whenever building anythinhg, make either an item or widget that the users uses to declare or remove "friend" from player, or maybe add it to the "Tab" screen.

I've seen a few mods with screen widgets about the kind I'm talking about, but I would point specifically to the "Waypoint" mod for that or, as I mentioned, using the tab screen, adding buttons to it.
http://steamproxy.net/sharedfiles/filedetails/?id=714735102

If you are able to do that, it opens up a wider array of possibilities for your mod, such as:
You can add "enemy" tag as well, for future uses.
Friends "share ownership" but you can create a wand or another method to set an specific item as "private" which friends don't share.
You can make it so that the player can choose if traps work for everyone or only against enemies.
In case of an internal fallout in a chaos server, players can remove ex-friends from their lists.
And so on.

Hope it helps and you find it interesting.
, \ /,,