Members of group in remote forest

Hi I'd like to make the following code work against a remote forest so need to take credential and DC/DN name.

It finds members of group and is much faster than get-distributiongroupmember

Add-Type -AssemblyName System.DirectoryServices.AccountManagement

Measure-Command {Get-ADGroupMember 'Domain Users' }

                $domain='DOMAIN'
$groupname ='GROUP'

               $pc = New-Object System.DirectoryServices.AccountManagement.PrincipalContext([System.DirectoryServices.AccountManagement.ContextType]::Domain, $domain)
               $group2 = [System.DirectoryServices.AccountManagement.GroupPrincipal]::FindByIdentity($pc, [System.DirectoryServices.AccountManagement.IdentityType]::Name, $groupname)
               $group2.Members.GetEnumerator() | % { $_.DisplayName }

Could you help where to add directory context and $credentials?

Thanks