To change the iPhone login name on your phone you tap Settings > General > Reset > Reset all settings. From there go to the app store on your iPhone and select an app to download and when you go to download it, it will allow you to input a new username and password. This will not erase any content ...

Hi max,
If you want to chat only with selected people then you just invisible your status. If you invisible your status then any other person con not see you are on-line or not but you can see all the peoples. Then you can chat with your selected person.