We consider wireless ad hoc networks with a large number of users. Subsets of users might be interested in identical information, and so we have a regime in which several multicast sessions may coexist. We first calculate an upper bound on the achievable transmission rate per multicast flow as a function of the number of multicast sources in such a network. We then propose a simple comb-based architecture for multicast routing, which achieves the upper bound in an order sense under certain constraints. Compared to the approach of constructing a Steiner tree to decide multicast paths, our construction achieves the same order-optimal results while requiring little location information and no computational overhead.