Blog users can' Post -Defualt User Roles With Blogs

Hi :

I am using wordpress 2.9.1 ,buddypress 1.1.3 with community theme

I have noticed that ,users those who created their own blog ( I am using Sub Domain) can't able to post new posts and pages from their dashboard.

I think the problem is lies with user roles , but being created his own blog the user has the privileges to manage their posts ( How can i do that automatically )

I want the users have author roles over thier own blogs and conributor role over parent blog ( it should be done automatically , i am aware of that using WPMU Default User Role we can do by manually, but i am looking for automated method)

Help me to get over this issue .