Instant Messaging for group chat

Dear all

I am looking for an instant messaging (IM) service that is well sutied for group chats. It should operate as a client/app - I am not looking for a service that requires you to log in via a browser. Microsoft Messenger is a good benchmark, but it ought to cater towards group chats rather then Messenger's primary one-to-one capability. It should also keep the history of chats that have been started, it should be possible to start several chats with the same group and it should be possible to have chats with several groups. Last but not least, it should run all the traditional platforms: iPhone, Android, PC, MAC etc., should be easy to install and require min. info to sign up.     

Does anybody know if such a client/app exists?

Best

M