bp_set_current_user_default_role()

function bp_set_current_user_default_role() {

	// Bail if not multisite or not root blog.
	if ( ! is_multisite() || ! bp_is_root_blog() ) {
		return;
	}

	// Bail if user is not logged in or already a member.
	if ( ! is_user_logged_in() || is_user_member_of_blog() ) {
		return;
	}

	// Bail if user is not active.
	if ( bp_is_user_inactive() ) {
		return;
	}

	// Set the current users default role.
	buddypress()->current_user->set_role( bp_get_option( 'default_role', 'subscriber' ) );
}