Chat windows are labeled as "Instant Message" windows. This document explains how to tell the difference between the two.
Symptom
Chat windows are labeled as "Instant Message with <user_name>".
Solution
An "Instant Message" window is for interaction between two and only two users. You cannot add new participants to an Instant Message window. "Chat" is the term that describes instant messaging between three or more users in the same window. You may notice that Chat windows are labeled as Instant Message windows, even though they are functionally different. To distinguish between the two, click the silhouette button in the message window to open the Participants drawer. If the add participant button (the plus sign) is available, you have a chat window. The add participant button is dimmed (unavailable) in an Instant Message window.
For more information, see technical document 107099, "iChat: Cannot Add User to Instant Message Window".