Mobile ad hoc networks (MANETs) are wireless networks that lack infrastructure. Security requirements in MANET have to be much efficient due to specific characteristics of ad hoc network. Intrusion detection system (IDS) is an integral part of any mobile ad-hoc network (MANET). It is very important for IDS to function properly for the efficient functioning of a MANET. In this paper, we try to build an IDS based on a co-operative scheme to detect intrusions in a mobile ad hoc network using basic game theory concepts. To identify the intrusion we deal with two layers - the application layer with underlying grid architecture and the network layer. We simulate a couple of intrusions in each of the two layers and look to resolve them using the coalition formation of the mobile nodes. These nodes obey the shapely value phenomena of the game theory concept and form coalitions to report intrusions in the network.