The net time command requires admin rights to run, so letting users run that command (assume you are wanting to put this in a login script) won't work.
You can, however, accomplish your goal with Group Policy. I'm actually getting ready to implement this where I work myself. I found that the previous network admin had the login scripts set to run the command you mentioned, BUT...users are non admins so this same error you are seeing would occur. Here's an article I found that gives instructions for the Group Policy solution:
Configure Windows NTP client GPO
Like I said, I've not implemented it myself yet, but plan to very soon. We have three domain controllers (Server 2008 R2), and have made them all NTP servers. One thing I need to decide is how I will implement this policy. I'm thinking I'll set it per organizational unit (OU) instead of at the domain level so I can split the load between the servers (we have nine offices, each is a separate OU).