The institution of group ownership has rarely been studied in political philosophy. Instead, most property theories in this discipline focus on arguments for and against individual ownership. When group ownership is discussed at all, moreover, this is often in the specific context of workplace governance. What is lacking is a general normative justification of group ownership that explains when and why it is valuable that individuals share a resource. This dissertation provides such a justification. The particular conception of group ownership that it defends is the institutional realisation of a sharing practice, called sharing in common. In this practice, members of a private group determine collectively and democratically how their shared object may be used. This conception of group ownership is irreducible to individual property rights, because under this institu-ion, individual rights to use and/or derive income from an object are authorised and subject to change by the group’s collective decisions. Group ownership so understood is valuable when and because it secures basic non-domination, and that it is also justified for that reason. People enjoy basic non-domination when they have the capabilities that are reasonably required to resist arbitrary power, and are equally in control of the decisions that affect these basic capabilities. This ideal constitutes the basis for a society of equals, where the normative status that belongs to beings capable of practical reason is affirmed for everyone. This dissertation develops two criteria for judging whether property institutions realise basic non-domination. First, they must promote people's ability to use resources to gain their basic capabilities. Secondly, the institutions must place the people who rely on a resource for their basic capabilities, in control of how that resource may be used. Using common property regimes (CPRs) in natural and agricultural resources as the central case study, the dissertation shows how existing institutions that approach the ideal of sharing in common can succeed on both counts. When democratically organised, CPRs al-low resource users to efficiently share and manage their resource themselves, and thus obtain their own livelihoods on their own terms. Other institutions that approach the ideal of sharing in common, such as insurance mutuals, energy cooperatives, worker cooperatives, and knowledge commons, are also briefly discussed for their potential to secure basic non-domination. The dissertation furthermore clarifies when and why alternative strategies for realising basic non-domination – such as through equal individual property rights, strong exit opportunities, and public ownership – fail to achieve that goal. The resulting theory not only defends a particular conception of ownership, but also provides tools that can facilitate research on ownership institutions and on basic non-domination more generally.