Despite having faced severe criticism in the past, mereological approaches to group ontology, which argue that groups are wholes and that groups members are parts, have recently managed a comeback. Authors such as Katherine Ritchie and Paul Sheehy have applied neo-Aristotelian mereology to groups, and Katherine Hawley has defended mereological approaches against the standard objections in the literature. The present paper develops the mereological approaches to group ontology further and proposes an analysis of group membership as parthood plus further restrictions. While all mereological accounts agree that group members are parts of the group, it has become clear that this analysis is insufficient. I discuss three proposals to develop the mereological analysis of group membership and then put forward a combined solution to the puzzle. According to my proposal, the members of a reading group are agents who are part of the group and have been designated to contribute to the group.