Agility in IT services or business is closely linked with the ability to quickly compose new offerings to match specific and varying demands. A modular architecture is a key enabler for this ability. In this paper we provide a brief introduction how the principle of modularity may be applied to business services. We then present a proposal how modular business services can be modeled using SeeMe, a socio-technical modeling language. The ability to compose and adapt services to new demands depends critically on the coupling of the modules. We show how SeeMe models of module interfaces can be used to assess coupling between modules. In the outlook to further research we propose that the concepts of vague modeling as well as person, activity, and resource hiding may be used to assess the agility of business services.