In this paper we present a CDN-based system, namely the COMODIN system, which is a media on-demand platform for synchronous cooperative work which supports an explicitly-formed cooperative group of distributed users with the following integrated functionalities: request of an archived multimedia session, sharing of its playback, and collaboration through questioning. The server-side architecture of the COMODIN system is organized into two integrated planes: the Base plane, which consists of a streaming CDN providing media streaming, and the Collaborative plane, which provides the collaborative playback service. At the client-side, the system centers on a Java-based application which interfaces the cooperative group of users.