We get that request a lot and it's definitely something we're looking to add. Thanks for pointing it out!
I see that this thread dates back to November of 2007 and the ability to add groups of authors to blogs hasn't been added. Are we ever going to get this functionality? More organizations are using Clearspace as an Intranet community and it would be so much easier on the admins if we could leverage group adding for blogs. Can you say if this is something that is even on the development team's to do list?