Do you have just one requirement you want to do this for? Do you expect to have other requirements that will need this same restriction applied?
Take a look at the Administrator Guide section on Workflow Customization. They have lots of examples. One example discusses restricting functionality based on a users group membership, I believe. (I don't have the document in front of me, so can't provide a more specific reference.)
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)