This is how I did it in VI 3.5 / VC 2.5
Click the Administration button in the VI client
Click Roles
Click Add Role
enter a name for the role, eg "storage admin"
Expand Host, then Configuration
Tick "Storage Partition Configuration"
Once that is done, go back to to the Hosts and Clusters view, and click the relevant host or datacenter, etc that you want to assign the permission to
Click the permissions tab
Right click and choose Add Permission
Choose a windows user or preferably group in the left hand side, and choose your new role (storage admin) in the right hand side
You may need to add other privileges depending on what they need to do, but start with just that one