There is a Group Blog plugin which would do what you are looking for.I’ve tested it a few months ago.It creates a blog in the group ceate/admin process then you would need to select a theme which would match with your main blog. Not sure if it works with bp 1.5 yet though,i tested it on bp 1.2.9 http://buddypress.org/community/groups/bp-groupblog/
I’m favouring Brunelleschi Theme at the moment. You can see it running at http://www.dennisbarkercv.com
compatibility is by running the buddypress buddypack plugin but it does pretty much what I want with no coding.