Mouse over your organization's name in the upper right corner and click Users. Select the user you want to change settings for and their row will be highlighted in blue. Then mouse over the "View/Edit" menu and click "Security Setting”. At the top of that page you will see two tabs, “Application Security” and “Site Access”. Click on the “Site Access” tab. This will take you to a list of all your Member Sites, you can then checkmark which sites this user will have access to.