Gmail has a built-in Google Talk gadget that lets you send instant messages to people in your domain, directly from your email application. You can also chat with people outside your domain using the standalone Google Talk client.
Send instant messages
Quick steps on using Google Talk to send instant messages and use voice and video chat. <<Edit if users won't be using the voice or video plug-in.>>
Find out more about using IM, voice, and video chat with the Google Talk gadget in Gmail.
Download the standalone Google Talk client to chat via IM, voice, or video with users outside your domain,