Copying Users between AD Groups with PowerShell

List the members of your source group:

Get-ADGroupMember -Identity "source_ad_group" | Select-Object Name | Sort-Object Name

Do a foreach-object to write every member of your source group to your target group

Get-ADGroupMember -Identity "source_ad_group" | Foreach-object {Add-ADGroupMember -Identity "target_ad_group" -Members $_.distinguishedName}