Group dynamic assessment (G-DA) grounded in Vygotsky’s socio-cultural theory (SCT) is believed to have the potential to provide a context for capturing a group of learners’ Zone of Proximal Development (ZPD) (Poehner & Lantolf, 2010; Shabani, 2010). The present study aims at testing the applicability of G-DA in identifying the mediational strategies offered by a mediator during his G-DA interactions with a group of L2 learners in the context of listening. Moreover, it seeks to unravel the effects of G-DA-based instruction on the co-construction of knowledge among L2 listeners. A microgenetic, longitudinal and interactionist methodology formed the theoretical basis for the construction of the assessment procedures. The participants included a group of L2 learners ranging in age from 20 to 25.The materials used in the assessment sessions were authentic texts selected from the normal VOA broadcasts. The qualitative analysis of G-DA protocols led to the development of an inventory of mediational strategies consisting of different forms of implicit and explicit feedback. The analysis also showed how collective scaffolding (Donato, 1994) could pave the way for establishing distributed help among learners (Platt & Brooks, 1994) within the social space of the class in the course of which secondary and primary interactants mutually benefit from each other's contributions (Poehner, 2009). The inventory of mediational strategies helped track the learners’ microgenetic and developmental trajectories over time. Finally, on the basis of the findings language teachers are suggested to use mediational strategies developed in this study as a reference to provision of feedbacks during interactions with a group ZPD.