Introducing dynamic aspects in diagrams that capture the aggregate structure of composite agents represents a promising notion, which can help to express in a straightforward manner how an entity is changing its placement during its evolution in time. This paper focuses on the novel concept of a dynamic aggregate "prototype" for agent-oriented modeling languages, which enables business modelers and system designers to play with and generalize models of existing reality. First, the importance of the role concept in dynamic collaboration is discussed and then the necessity of having explicit hierarchical structures for agents is argued. Next, the rationale for introducing agent groups is presented and their relation with the role concept is explained. A comparison with other approaches that offer agent aggregation is done. Finally, an example is used to illustrate the use of the new concept, and future avenues for research are introduced.