While implementing participation in an organisational health intervention (OHI), the formation of a group of representatives responsible for developing and realising action plans is a common approach. This mixed-methods study aimed (a) to examine differential intervention effects for employees who are directly involved in intervention decision making (direct participants [DPs], N = 84) compared with the remaining employees (indirect participants [IPs], N = 99) and (b) to explore the transfer process between these groups. Quantitative surveys were collected at two time points (follow-up after six months), and four focus group discussions were conducted during the follow-up phase. The results show that DPs experienced an improvement in intervention outcomes (psychosocial working conditions and affective states at work) compared with IPs. For IPs, no positive change was observed. However, further subgroup analyses found that IPs in “successful teams” were also able to experience improvement in intervention outcomes. Qualitative focus group data gave insights into the beneficial and less beneficial transfer process mechanisms between DPs and IPs that could explain these differential effects for IPs. This study highlights the importance of considering the forms and quality of participation in OHIs and offers insights into the processes shaping the intervention effects for whole teams.