[Paul Wong-Gibbs]

BuddyPress can run on a sublog. All you need is this in your wp-config.php:

define ( 'BP_ROOT_BLOG', 2 );

Will make BuddyPress run on blog ID 2, for example. For other handy commands, see https://codex.buddypress.org/how-to-guides/changing-internal-configuration-settings/