This paper addresses the concept of participation in the context of design and transformation of a health information infrastructure. Recent work on transformation of health information infrastructures shows that bottom-up user-driven processes are preferred, however, the role of the user in infrastructuring remains under-researched. We investigate user representatives' space for participation in design of an infrastructure, based on the Scandinavian tradition of Participatory Design. We take a user perspective to infrastructuring, investigating the work of user representatives (health workers) who engage in technology design, requiring engagement with local practices in shaping data practices. We have conducted a longitudinal case study on the design and development of a data platform for patient handover in a large Norwegian municipality. We contribute with an understanding of participation in the transformation of infrastructures as user representatives engage local organizations as both facilitators for their own design process, i.e. designers preparing and transforming their local services, as well as participants into design of the infrastructure, i.e. designers shaping technology. We contribute to literature on infrastructuring by showing how user representatives explore possbilities for action, shaping the infrastructuring process.