i am thinking to have it like, login screen ->enter button->show page with online users->clik on any one online user->move to new page and start chatting.
Are you a beginner at Android, PubNub or programming in general?
i have basic knowledge of android, but nva worked with any api
And return to the PNDC with more questions you have as you attempt to build your first PubNub Chat App. Best of luck.
look at my https://play.google.com/store/apps/details?id=eu.fotodar.wwj it is a web server with chat, messages,sharing & custom.web pages