Quick and dirty.
This uses the memberOf attribute on the computer object and then filters client side (Where-Object) for those that aren't in the group.
I recommend setting the searchbase and using the distinguishedName for the group.
Get-ADComputer -Filter * -SearchBase "OU=Computers,OU=xxx,DC=xxx,DC=yyy,DC=zzz" -Properties MemberOf, Description |
Where-Object {"CN=groupA,OU=Groups,OU=xxx,OU=xxx,DC=xxx,DC=yyy,DC=zzz" -NotIn $_.memberof}
| Select distinguishedName, Description