How do I make Google Voice's SMS service my primary SMS service?

I just recently started using Google Voice. The application is installed and I have it set up to receive SMS messages and send them to my phone.
What I want is to be able to use Google Voice’s SMS service as the primary service for certain people and not for others. Is there a way I can select whose SMS messages get sent through Google Voice and whose don’t, or to select a certain group of people whose SMS messages will be sent/received with Google Voice?
(I’m so sorry if this seems confusing. If anyone needs me to clarify anything please ask.)