If i understand you correctly, all you have to do is to place a map_entry_point at the edge of the playable area of the map, and inside of the territory, and the owner is set to world.
when captured you will be able to call in your units from this point.
Remember units will arrive from the closest map_entry_point to your rally-point.
You can also use map_entry_point_player if you of some reason wants to limit the use to the player that captures the point in a team.
Edit: Janne252 you did beat me to it while i was typing =P
for i=1,World_GetPlayerCount() do local player = World_GetPlayerAt(i) Player_CompleteUpgrade(player, BP_GetUpgradeBlueprint("allow_building_campfires")) end