The group chat feature is available for anyone using the Google Talk network. Some of your contacts, including those connecting through a federated network like Earthlink, Gizmo Project, Tiscali, Netease, Chikka, and MediaRing, won't be able to participate in a group chat. You'll still see their addresses in your Chat list, and you can still chat with each of them one to one; however, when you invite one of them to a group chat, a message appears stating that the person cannot be invited.
You can invite any of your contacts to join a group chat, regardless of what chat client they're using. An invitation will appear in their chat window.