Here is how you get this to work:
1) Accept the Rogue Robots quest and travel to the destination.
2) Kill the robots and complete the quest. Settler speech options ONLY open up after you complete the quest.
3) Talk to the settler. The first one you talk to offers to join your settlement. Send settler to settlement
4) All other settlers will act like they are part of your settlement after this. You cannot send buy one each time.
To those who have asked, I am using the Awesome CRAP for Settlers PC mod to give my settlers unique lore-friendly clothing:
http://www.nexusmods.com/fallout4/mod...
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