A large number of network applications today allow several users to interact together using the many-to-many service mode. In many-to-many communication, a session consists of a group of users (we refer to them as members), where each member transmits its traffic to all other members in the same group. In this paper, we address the problem of dynamic provisioning of optical WDM networks to support many-to-many traffic grooming. The objective is to minimize the overall blocking probability of arriving many-to-many sessions. We address the problem in both non-splitting networks where the nodes do not have optical splitting capabilities and in splitting networks where the nodes have optical splitting capabilities. In each of the two networks, we propose a number of dynamic provisioning heuristics and we provide extensive experiments to evaluate and compare their performance.