Abstract
This is the protocol for a Campbell systematic review. The main objective is to explore the general efficacy of group-based community interventions aimed at supporting marginalised adults with mental illness and related problems on outcomes such as problem behaviour, subjective well-being, homelessness, poverty and employment. Furthermore, the objective is to explore the potential advantages/disadvantages of using a group-based versus an individual intervention when targeting specific problems or when using specific types of interventions.