BuddyPress.org

You can indicate to bp that it should consider a particular blog/subdomain as the one it should operate on. That is defined by the BP_ROOT_BLOG in bp-core.php. Change that to the blog id that you want bp to consider it’s own. In bp-custom.php:

define( ‘BP_ROOT_BLOG’, X );

Where X is the blog id of your subdomain.

I’m not sure what you mean by ‘buddypress installed in a subdomain’. There are VPS defined ‘subdomains’ done at the server level and there are wpmu’s concept of multiple blogs as wildcard ‘subdomains’ mapping to a particular blog id.

bp is installed wherever wpmu is installed. It has no independent relationship to domains. bp only has knowledge of blog ids.