Having read the email, I'll expand my reply here a little bit.
When you have the s2Member Pro add-on, you'll be able to activate the ClickBank gateway.
WP Admin -> s2Member -> Other GatewaysThenk you'd configure the integration from the options page, which includes instructions on what to do in your ClickBank admin page.
WP Admin -> s2Member -> ClickBank OptionsYou wouldn't be able to prevent users from sharing their login info, but you can control how many IPs get access to the account before getting locked out and needing to contact you. This is not a Pro feature and you can check it out in the restrictions page.
WP Admin -> s2Member -> Restriction Options -> Unique IPsFile downloads can be limited to a number of downloads depending the user's level, and only logged in users with the right access would be able to download the file. Sharing the link to the file would not work unless the person trying to download were logged in with the right access.
WP Admin -> s2Member -> Download OptionsI hope that helps.