Please note that when you accepted the raiders' offer, you promised to share the looted goods with them. It is better to choose wisely and equitably, because otherwise raiders might want to pay a visit to your base and take what they believe they deserve. Moreover, thus you are also disclosing the coordinates to other survivors, who might be willing to take a share of your precious belongings. 
To summon the raiders, your character must be at level 150.