In group chat once user goes offline, he also goes out from group. How can I kept him in group so that he will receive chat notification as well when he comes online. I am using ejabberd and strophe js for web chat application.
Multi-User Chat as defined in XEP-0045 is based on presence. Thus, that's the expected behaviour.
ejabberd also implements a work in progress extension written by the XSF: XEP-0369: Mediated Information eXchange (MIX).
ejabberd master implements that specification already even if this is just a draft. At the moment, it is not finished and is just experimental. It cannot be used in production just yet.