MANET security has long been an issue for its infrastructure-less nature. The sort of networks gives rise to authentication and key-management problems in MANETs. The absolute TTP based schemes seem to be infeasible for being adopted in MANETs and so are the non-TTP based self-organized schemes which suffer security problems regarding key identity of nodes. A hybrid scheme can serve the purpose. One of the hybrid schemes addressed these issues effectively by pre-assigned logins on offline basis and issuing certificates being online, using 4th generation services. However, the scheme revealed overheads in different scenarios. We have proposed a scheme where the nodes having common interests, once authenticated through external resources, form a group with a single key shared for the whole group. The overlapping nodes sharing different groups facilitate authentication of other nodes communicating from those groups. In this way the external messages for authentication among the nodes of different groups can be reduced to a great extent. Denial of Service (DoS) attacks are the major problem in the MANET, because the attack consumes huge bandwidth and resources. The proposed mechanism also handles the DoS attacks.