This paper proposes a MANET/VANET based group communication system, P2Pnet, to support the communication and information demand for mobile group users such as rescue teams in catastrophic disasters, army in battle fields, and mobile learning groups. We use the Jiji Earthquake as an example to illustrate the physical environment and to establish system requirements for a mobile group communication system that are suitable for the environments mentioned above. We have implemented the first phase prototype as well as two application experiments on top of P2Pnet: Rescue Information System for Earthquake Disasters (RISED) and a mobile English learning experiment conducted in the Nation Chengchi University. Keyword: Mobile Computing, MANET, P2P Computing.