You must be a Bionic Administrator to adjust security settings.
To set up security, you will need to click through on the Advertiser you wish to modify.
Click on the Settings view.
Select the Security section.
By default, security will be set to Open.
To restrict access to this Advertiser, select Restricted.
Press the Add Users button.
Select the users you wish to have access to this Advertiser (and all of its sub-Advertisers).