Source

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

public function chgrp( $file, $group, $recursive = false ) {
		if ( ! $this->exists( $file ) ) {
			return false;
		}

		if ( ! $recursive ) {
			return chgrp( $file, $group );
		}

		if ( ! $this->is_dir( $file ) ) {
			return chgrp( $file, $group );
		}

		// Is a directory, and we want recursive.
		$file     = trailingslashit( $file );
		$filelist = $this->dirlist( $file );

		foreach ( $filelist as $filename ) {
			$this->chgrp( $file . $filename, $group, $recursive );
		}

		return true;
	}