How to invite people or friends to chat in Gmail?
You can chat with your friends or people in the gmail just with their mail address by using gmail hangouts.
Step 1: Click the search icon from the left pane or click Start a new one.
Step 2: Your mail contact will be listed here.Search the contact in the search box or click the contact you wish to chat.
Step 3: Chat message pop up opens for the selected mail address. Type the required message in the text area as given in the image below.
Step 4: Hit enter once you typed the message.
Step 5: In the left pane, chat users will be listed. People who stays online will be shown with the green round near their profile picture. You can chat with the person who stays online.
Note: While staying offline, you can still view the chat messages if you go online.