Tele-collaboration between medical professionals is well established for specialties like radiology, cardiology or pathology. Typically these applications do not allow for ad hoc participation of non-registered users like patients and / or medical professionals. This paper describes an approach for extending these tele-applications to non-registered users and providing ad hoc participation with means for providing and accessing data. A light weight, web-based approach is used to assure acceptance and ease of use while maintaining and complying to the required regulations regarding authentication, authorization and protection of personal data. The approach has been implemented and is in use with a nationwide tele-collaboration network in Germany.