Is there a group therapy for people who lost a loved one who overdosed?

Support/therapy grps. Are available in every major city for many types of loss & distress. Call a local mental health agency or professional for specific information.
Yes. I'm sorry for your loss. There are many support groups for people in this very painful situation. The american foundation for suicide prevention has much information in general, and also a lot about surviving suicide loss: http://www.Afsp.Org/index.Cfm?Fuseaction=home.Viewpage&page_id=742a015c-d811-979a-ab84379c813f8d93 they also have a link listing many support groups.