We propose an efficient reachability estimation scheme for group membership services in mobile ad hoc networks (MANETs). The periodical message exchange-based scheme, i. e., a typical reachability estimation scheme, requires message exchanges even when the reachability status does not change. It is presumed that the reachability between nodes is maintained while the nodes move around in a limited range. The proposed scheme exploits a virtual grid for the course-grained estimation. A region in the virtual grid can be used to represent the movement range which does not change the reachability. Each node calculates how long it will stay in a region, and issues the duration information only when it gets out of the current region.