Source

File: wp-admin/includes/class-wp-filesystem-direct.php

public function group( $file ) {
		$gid = @filegroup( $file );

		if ( ! $gid ) {
			return false;
		}

		if ( ! function_exists( 'posix_getgrgid' ) ) {
			return $gid;
		}

		$grouparray = posix_getgrgid( $gid );

		if ( ! $grouparray ) {
			return false;
		}

		return $grouparray['name'];
	}