If you have an AD group full of members, and you want to copy that memberlist over to another group, then it can be done with this powershell scipt:
From: put in the AD group where you want to copy users from.
TO: put in the AD group name where you want the users to be copied to.
Get-ADGroupMember “FROM” | get-aduser | foreach-object {add-adgroupmember -identity “TO” -Members $_.SamAccountname}
Thats it.