In fallout 4 it'd make sense if you can move all your settlers to one 'main' base rather than having everyone spread out, as no one location has enough settlers to run all the stores etc.
How do I tell settlers to go to another settlement?
Moving settlers from one settlement to the next works just like setting up a supply line, only instead of choosing SUPPLY LINE
you need to choose MOVE
.
Image originating from @kalina's answer linked above.
After that, you'll get a list of settlements to choose from.
It is possible that the settlement you want to send them to is grayed out, however. This can happen when the settlement is full.