Well..it depends on the NPC. Most of them stay where they are and wait for some other nice party to take them in
Which NPC's were you having trouble with? For Khalid and Jaheira...why wouldn't you take them in?! For most of the game they are pretty good..if you say no to them I think they just stay in the same spot until you say yes. Almost all of them wait in the same spot until you come back. Although..some do go back to where you found them. If you say no to Gerrick (say no..he sucks) than he goes into the Red Sheaf Inn...right next to where he is standing. If you go in there he is not there...but if you go back out POOF! there he is. Kagain goes back to his shop when u dump him. Most of them stay in the same spot.