UX wise, I figured that the user will travel around with a 3rd party cookie from our main domain. The site operator will install a simple <script> tag. The scri

UX wise, I figured that the user will travel around with a 3rd party cookie from our main domain. The site operator will install a simple <script> tag. The script tag will fire off a jsonp request to our server. If there's no cookie present, the resultant js will either do nothing, or at the site operator's discretion invite them to join our system. If the cookie is present, and a valid session is detected, and they haven't logged in in the last 24 hours the resultant js will popup a small "notification" (or just use html5 notifications if available) that you've checked in. If there's a cookie, but no session, the js will, at the site operator's discretion, either do nothing or invite them to login (inline) and claim their checkin. I'm not sure what you mean by relationship, but we will know who the user is and what the site is, so we would obviously be tracking a relationship there. Thanks for the feedback!