Save membership of source user into a variable:
$CopyUser = Get-ADUser HansMuster -prop MemberOf
Save membership of target user into a variable:
$CopyToUser = Get-ADUser SeppMuster-prop MemberOf
Copy group membership:
$CopyUser.MemberOf | Where{$CopyToUser.MemberOf -notcontains $_} | Add-ADGroupMember -Member $CopyToUser
