How to secure feeds

I know the link "http://inedo.com/support/documentation/proget/feeds/api-keys" but this does not tell me how we can assign groups or users to the feed or not.

How can we do that?

Product: ProGet
Version: 3.3.11

You can do this by granting/denying privileges for specific feeds, which is accessible on the Admin->Privileges & Roles page, documented here: http://inedo.com/support/documentation/proget/security

Unfortunately I don't see that option

Please note that granular, feed-based security is only available in the paid Enterprise edition. In the Express edition, privileges may only be applied to the whole system. If you would like to trial these features, you can request a trial key at https://my.inedo.com