However, the most recommended method to make these legal agreements enforceable is called the clickwrap method. You should always link to the legal agreement page right next to the checkbox to make it easier for users to read the agreement.
The same checkbox method should apply to already registered users of your website, mobile app, SaaS app , Facebook app etc. This is how Elance shows a notification on their updated Terms agreement to their registered users:. Microsoft does something similar by notifying users of a Terms of Service update affecting their accounts. In this case, use PHP or the language your website is programmed in — Ruby, Phyton — to add a second layer of checking.