Background. Involving people with acquired brain injury in service development has the potential to improve service and give experience-based knowledge legitimacy. The objective of this study was to explore experiences of sharing experience-based knowledge of living with acquired brain injury with others, with a particular focus on the conditions for sharing and learning, and the legitimacy of experience-based knowledge. Materials and Methods. Using a single case study design, the processes in two groups that were part of a Swedish supportive network for people with acquired brain injury were explored. One group consisted mainly of people with acquired brain injury who authored a blog and the other group consisted of healthcare staff who produced educational material with the involvement of a person with own experience of acquired brain injury. The data consisted of forty-one collective blogs, fifteen semistructured individual interviews, and ten observations from meetings. The data were analysed by utilizing the community of practice framework and the concepts of legitimate peripheral participation and epistemic injustice. Results. The findings showed that both groups developed learning processes with a focus on everyday rehabilitation as a joint enterprise. Mutual engagement developed from doing activities together and legitimacy in the groups came from engagement in these activities. In the education group, the ambition to involve people with own experience of acquired brain injury was never realized in practice. Hence, experience-based knowledge of living with acquired brain injury never got legitimacy in the group. Conclusions. We conclude that integrating experience-based knowledge from people with own experience of acquired brain injury demands careful and deliberate planning with specific consideration to existing power asymmetries between healthcare professionals and people with own experience. Mitigating epistemic injustice and gaining legitimacy for such knowledge require that people with experience of living with acquired brain injury are recognized as knowledge producers. © 2023 Lill Hultman et al.