Nowadays large scale data storage and processing can be achieved on a set of federated computers which can be interconnected over a LAN (local area network), a cluster or even a grid. Many researchers attempted to achieve this work on volunteer grids where the corresponding computers are voluntary connected or disconnected. In fact, this kind of grids presents several advantages such as the possibility of the simultaneous utilization of the corresponding computers by different users or/and applications. The big challenge of such grids is how to guarantee to their users the trust and the required security. In this paper, we show how identity federation based on agent technology can be exploited to deal with this mentioned challenge.