Or one could just read the manual
Top of page 55: "These Non-Player Characters (or NPCs for short)...."
Bottom of page 60: "When NPCs join your party, they become RPCs (Recruitable Player Characters)....
Also, you can take any RPC anywhere, if you really want to. But you'll have to listen to their whining the entire time you're in an area they don't like. Some of the complaining (Saxx, RFS-81) is easier to tune out than others (Vi).