Currently, non-state armed groups (NSAGs) are bound by customary international humanitarian law (IHL) without their consent or inclusion in its making. On the one hand, NSAGs’ inclusion in law-making would make IHL more adapted to situations of rebel governance, the capabilities of NSAGs, and could result in a compliance pull vis-à-vis NSAGs. On the other hand, such inclusion might be undesirable, unreasonable, and unfeasible. Nevertheless, all arguments pro and contra NSAGs’ inclusion do not necessarily apply to all types of NSAGs. However, scholarship lacks a convincing criterion to decide which NSAGs could participate in the formation of customary IHL. This article attempts to provide such a criterion by approaching the discussion in a non-state-centered manner and puts those most affected by armed conflict at the center: communities under rebel governance. It proposes a more holistic interpretation of the practice-criterion for custom-making in combination with the criterion of internal legitimacy to decide which NSAGs to include in customary IHL’s making. Internal legitimacy is generated when communities consent and support rebel governance in a non-coerced manner, motivated by performance-based and symbolic catalysts for legitimacy. This article elaborates on what is understood by the notions of procedural fairness and non-coerced community consent and support; and how it answers the concerns about delegitimizing national governments, legalizing armed struggle, and the detrimental impact of NSAGs’ practice on the substantive protection offered by IHL.