What you need is Handcent SMS
(Free). This pup allows you to set up Group's by Business, Coworkers, Family, Friends, etc.
I guess I better outline how to setup a Group!
1. Goto GMAIL
2. Select "Contacts
" you want for your Group
3. Select "Groups
" (on the right hand side of your screen.
4. Select "New Group
" and give it a name.
5. Now, the next time you Droid sync's with GMail - you will have that Group added.
6. Now, back to your Droid - Select "Handcent"
7. Start a new Text Message
8. Hit the "+
9. Select "Groups
10. Select the Group you want to send your Text Message to...
11. Hit Menu and select "End Select" and you will return to your Text Message.