I have asked a few of my friends to join this website and help it improve itself. But I wanted to contact them via this website, of course by chat. But how can I start a chat and invite users?
If you're interested in knowing how to invite ppl to a chatroom, take a look at this answer.
If you want to chat privately, I'd recommend using Discord or Skype (or whatever service you prefer) as all the messages are visible to everyone (and also indexed by google) on chat.stackexchange.com.