I. Introduction
Implicit human–computer interaction [1] is a typical human–computer interaction mode for pervasive computing. Computer systems analyze users' actions and intentions based on multimodal sensor data, and further provide attentive services to users without drawing their attention away from their current tasks. In intelligent environments such as smart meeting rooms, attentive services may include online archiving of meeting data, dynamic control of active sensors, and intelligent broadcasting. Such services can only be provided if computers understand current and previous group interactions that have been taking place inside the meeting room.