Here's a version of the ds command I found more typically useful, especially if you have a complex OU structure and don't necessarily know the full distinguished name of the group.
dsquery group -samid "Group_SAM_Account_Name" | dsget group -members -expand
or if you know the CN of the group, usually the same as the SAM ID, quoted in case there are spaces in the name:
dsquery group -name "Group Account Name" | dsget group -members -expand
As stated in the comments, by default the ds* commands (dsquery, dsget, dsadd, dsrm) are only available on a Domain Controller. However, you can install the Admin Tools pack from the Support Tools on the Windows Server installation media or download it from the Microsoft Download site.
You can also perform these queries using PowerShell. PowerShell is already available as an installable feature for Server 2008, 2008 R2, and Windows 7, but you'll need to download the WinRM Framework to install it on XP or Vista.
To get access to any AD-specific cmdlets in PowerShell you will ALSO need to perform at least one of the following installs: