If I create a group on my MSN, Yahoo messagers, etc. where many people can talk among them, is the group called "pool"?
I have never heard such a group called a pool, and I would not do so myself. A pool, in respect to people (and other things, such as cars or other resources) generally means a group whose members may be drawn, at need, to do some task, and who are not assigned to a permanent user or employer: a car pool, a secretarial pool.
It also has various senses, in respect to groups of people, in certain games, sports or in gambling, but I don't think you can easily extend these meanings outside of their contexts.