The user needs to get oriented and learn how to use or interact with a new site, application, or experience for the first time (or after a long time away).
The user sends an invitation to a friend or group of friends to have them join in a site experience.
The user receives an invitation from a friend or connection to join a site.
A user with an account but who is not currently signed in wants to participate by contributing something.
Users often don't notice TOS checkboxes and get harassed by error messages.