Re: Buddypress on a Subdomain
Give me the weekend to test a theory. This should be possible by setting the BP_ROOT_BLOG to be the ID of the blog you want it to be, but if it’s not working that way I need to take a look at why.
You’ll want bp_enable_multiblog set to false. Setting it to true allows all subdomains to have access to the directories and profiles areas. This is mostly useful if you’re trying to use specific subdomains for specific pieces of BP, or if you want all blogs/subdomains/sites to have access to the directories from within themselves.
If you take a look at profiles.wordpress.org, you’ll see how they have BP used there within a subdomain.
I also think the better answer to your question would be to create a custom theme that only shows BP elements in the domain/subdomain that you want to show them. But, that takes a little more work I suppose. 
