This tutorial was created using the vSphere client, but the general steps should pretty much the same for the Virtual Infrastructure Client.
If you are using vCentre please read the notes relating to this at the bottom of the article.

Create Privileges

1. Click "View | Administration | Roles"
2. Right client and cick "Add"
3. Select a name and select the required privileges

Create User

1. Click on the "Users and Groups" tab
2. Click on the "Users" button
3. Right click and select "Add"
4. Specify the desired User Name, Password, etc and Click "OK"

Create a Local Group

1. Click on the "Groups" button
2. Right click and select "Add"
3. Enter the group name you want and enter the User Name you created above in the User Name field and click Add
4. Click "OK" to create the group

Assign Permissions

1. Click on the "Permissions" Tab
2. Right click and Select "Add Permission"
3. Click on the "Add" button and select the Group you created above and click on the Add button.
4. Click on the OK button.
5. Choose the Assigned Role (Priviages) and click "OK".

Note : You can use the permissions tab in either the main inventory (main page) or per Virtual Machine. This is useful to know if you need to allow one user to access just one Virtual Machine.

vCentre

When administrating users on a single ESX box the users and groups are managed locally. With vCentre all users and groups are managed via the use of an Active Directory (LDAP) server. You can then specifiy which users and groups you wish to grant permissions to within the vCentre GUI.