This article seeks to apply the informal learning theory of guided participation to the formal primary L2 classroom. Utilising the concept of guided participation, in which the process of learning is defined as a collaborative interaction between expert and novice leading to a transformation of participation in community activity, the author analyses the respective mediational roles of teachers' guidance and peer assistance. In addition to the identification of transformation in participation, the microgenetic processes by which individual learners' L2 use emerges are also traced. This study reveals how active language use is associated with participation and changes in behaviour during microgenetic development. In order to understand this transformational process of language learning manifest through evolving engagement and language use, the author suggests the notion of "transformational linguistic participation." This construct also draws attention to the importance of affordances: complementarity of task design, pupils' perception and utilisation of linguistic challenge, interaction and the discovery of purpose in tasks. The author argues that application of a guided participation perspective to the L2 classroom highlights the social nature of the learning process, drawing attention to how learner engagement is created, learner agency facilitated and linguistic challenge promoted.