VANET applications are initially for safety purpose. Now, infotainment applications with group communication among vehicles are in the lime-light. In this research, we propose an overlay construction scheme to support multiple group communication in hybrid VANET. More specifically, we deal with group isolation which has a great impact on overlay reliability. By incorporating 3G access, the limited DSRC transmission rage is complemented with long range transmission nature of infrastructure-based 3G network. With an initialized overlay in DSRC manner, we propose 3G management scheme, including 3G peer list maintenance, upstream helper selection and helper discovery via overlay multicast. By taking advantage of the feature of tree-based overlay, the helper discovery scheme is triggered by isolated group members on demand. After helper discovery process, the helper selection scheme is also discussed. There are two most common situations leading to group isolation in VANET environment. The first one is group separation at intersection, and the other one is group separation at traffic light. In both situations, group members are suddenly split into two or more subgroups. The simulation result based on the two situations under different group model shows good overlay sustainability with proposed scheme.