bp_groups_get_activity_group( integer $group_id )

function bp_groups_get_activity_group( $group_id = 0 ) {
	// If displaying a specific group, check the activity belongs to it.
	if ( bp_is_group() && bp_get_current_group_id() === (int) $group_id ) {
		$group = groups_get_current_group();

		// Otherwise get the group the activity belongs to.
	} else {
		$group = groups_get_group( $group_id );
	}

	return $group;
}