Effectively utilizing groups in delay tolerant networks (DTNs) can both improve the throughput of unicast routing protocols and open the door for a wide range of paradigms, such as anycast and multicast. Unfortunately, in DTN environments, there is no centralized entity that can quickly and reliably transmit group membership lists, and hence group information must be disseminated through unreliable and potentially malicious nodes. In this paper, we propose a local and robust group information dissemination and consolidation protocol, called MembersOnly, that both quickly and accurately transmits group membership information to all nodes in the network, even if multiple malicious nodes attempt to disrupt the process. We show via analysis and simulations that MembersOnly is able to withstand multiple types of attacks, with only very limited periods of vulnerability that disappear relatively quickly. This is in contrast to current techniques that cannot withstand many of these attacks, resulting in quick and thorough corruption of group membership lists. In addition, we show via simulation that even the most basic routing protocols can gain a performance advantage when using MembersOnly.