bp_blogs_add_user_to_blog( int $user_id, string|bool $role = false, int $blog_id )

function bp_blogs_add_user_to_blog( $user_id, $role = false, $blog_id = 0 ) {
	global $wpdb;

	// If no blog ID was passed, use the root blog ID.
	if ( empty( $blog_id ) ) {
		$blog_id = isset( $wpdb->blogid ) ? $wpdb->blogid : bp_get_root_blog_id();
	}

	// If no role was passed, try to find the blog role.
	if ( empty( $role ) ) {

		// Get user capabilities.
		$key        = $wpdb->get_blog_prefix( $blog_id ). 'capabilities';
		$user_roles = array_keys( (array) bp_get_user_meta( $user_id, $key, true ) );

		// User has roles so lets.
		if ( ! empty( $user_roles ) ) {

			// Get blog roles.
			$blog_roles      = array_keys( bp_get_current_blog_roles() );

			// Look for blog only roles of the user.
			$intersect_roles = array_intersect( $user_roles, $blog_roles );

			// If there's a role in the array, use the first one. This isn't
			// very smart, but since roles aren't exactly hierarchical, and
			// WordPress does not yet have a UI for multiple user roles, it's
			// fine for now.
			if ( ! empty( $intersect_roles ) ) {
				$role = array_shift( $intersect_roles );
			}
		}
	}

	// Bail if no role was found or role is not in the allowed roles array.
	if ( empty( $role ) || ! in_array( $role, bp_blogs_get_allowed_roles() ) ) {
		return false;
	}

	// Record the blog activity for this user being added to this blog.
	bp_blogs_record_blog( $blog_id, $user_id, true );
}