if you use a network install, no matter as subdomain or sub directory, it’s preferably to install BP on the main blog.
On a standard MS, this means that WP is at the server root (ie. /xxx/xxx/htdocs/www/wp-content/)and BP installed as plugin in /wp-content/plugins/buddypress/ and network activated.
Don’t install BP on each blog nore on the main blog and a network blog…. Installing BP on one of the sub blog, not the main blog, will be complex and very complicated in some case to manage with permalinks and redirections, except if you know what you’re doing, and specially if you use sub domain blogs.