Task Models describe how to perform activities to reach users' goals. Task models represent the intersection between user interface design and more systematic approaches. Task models can be represented at various abstraction levels. When designers want to specify only requirements regarding how activities should be performed, they consider only the main high-level tasks. On the other hand, when designers aim to provide precise design indications then the activities are represented at a small granularity, thus including aspects related to the dialogue model of a user interface (which defines how system and user actions can be sequenced). In this paper a comparative analysis of selected models involving multiple users in an interaction is provided in order to identify concepts which are underexplored in today's multi-user interaction task modeling. This comparative analysis is based on three families of criteria: information criteria, conceptual coverage, and expressiveness. Merging the meta-models of the selected models enables to come up with a broader meta-model that could be instantiated in most situations involving multi-user interaction, like workflow information systems, CSCW.