Depends on the NPC type. Some you just talk to, others you need to place inside the facility they are needed in, like a factory, then use the options in Building Management. In all cases you must be at their location and they must be assigned to you.
They're added slowly - you might see something like the Mayor or Reporter be implemented in the coming months, but you certainly won't see any Wookiee bodyguards killing enemies for years to come.