This chapter describes the reasons why patients with a rare disease or parents of a child with a rare disease joined a patient organization. The guiding hypothesis is people join an association to access a good that they cannot find elsewhere, and the nature of which has an impact on the group. The interviewees highlighted four reasons to join a group, corresponding to four types of goods. Being a consumer means that the organization is a service provider (private good), being efficient means that the individual strives to contribute to the production of a public good, being active makes the organization a witness to the dedication of the individual (common good), being together means the group is an end in itself (club good). This fourth reason is widespread, suggesting that rarity could be an important driving force behind the vitality of rare diseases organizations.