Glancing consists of a server which runs centrally, and desktop clients.
The server runs an IM buddy dedicated to the group and a small webserver. It keeps track of the glance state and the group model. You can read about its technical implementation.
The clients are extremely small desktop applications which also have dedicated IM buddies running inside them to communicate with the group avatar. You can read about the prototype client.
To make sure members are allowed to join a group, they share a "secret" (just a few words) which is used during authentication. At the end of authentication, the client is given a session key.
A glance is done by the client making a www request to the server, using the session key.
Glance states are announced by instant messaging from the server to the client.
The server also monitor the presence of the clients using an internal buddy list. In this way it can keep track of who has gone offline.