This paper analyses the heterogeneous effects of membership of a farmer group on access to water, use of inorganic fertiliser, household incomes, and farm asset holdings. A sample of 401 irrigators in South Africa was analysed using propensity score matching. The study found that group membership had a positive effect on all four outcomes. Group members had an extra four days of access to water in a month, and applied at least 130 kg/ha more inorganic fertiliser, than non-group members. Group members had a higher household income per capita and more assets than nongroup members. However, the result revealed a heterogeneous effect among group members, with the benefits varying according to members’ socio-economic characteristics as well as internal group dynamics. The government and private donors should continue to promote the formation and organisation of farmers into groups. The role of group membership in farming outcomes can be enhanced if smaller groups are promoted. It is also crucial that strategies for promoting trust, reciprocity and group commitment be implemented for better group outcomes