I'm just wondering if there's any more named NPC's that can join your settlements, and where they are. I only know about the Vault Tec guy and Sheffield.

There are more, but a lot of them seem to be buggy or even outright broken.

Here is a list of merchant ones I found.

http://i.imgur.com/DhdcfyN.png

there's a trick I learned.

There are two spots where there are almost always guaranteed to spawn special encounters. The one that easiest for me, is Starlight Drive in, under the overpass where you fast travel at. Some of these encounters are special settlers.

Thanks guys. I actually just ran into Doc Anderson near Concord, facing the museum if you turn left and go down the hill there's a flat piece of land that I've seen settlers round a campfire, or wild dogs, and even raiders. But this time Doc Anderson was there, so I recruited her and sent her to The Castle.