Do you mean have a certain ringtone play for certain individuals within your contacts list? If so, you should be able to do so by editing the profile of the individual you want to assign a ringtone to in your contacts. You should see some sort of option that allows you to go through and select whatever ringtone you want for that person.
You can assign a ringtone to every contact in your contact list. A ringtone is the sound you hear when someone calls you. (Not possible on WP7 right now, though). You can't simply assign a ring back tone] unless you have enabled that feature through your provider. But as CompTIA_rep said you'll have to pay for that service. Usually a one time activation fee and then a monthly fee for the service.