There's also a group chat feature powered by Google Talk/Chat. It's a simplified version of the chat feature from Gmail, Google+, iGoogle and orkut. There's only one chat box that lets you talk with the other signed-in users that edit or just view a document, presentation or any other file. For some reason, it doesn't work in Google Sheets/Spreadsheets.
"To begin a group chat with everyone viewing the document, click the Chat button, which you'll find in the top-right corner of your window. A chat box will appear at the bottom of your file, and everyone who's viewing the document and who's signed in with a Google account will be included in the group chat," explains Google.
The group chat feature from Google Drive doesn't support voice or video chat, conversations aren't saved in Gmail and they're not available in other services that use Google Chat. "If you've chatted in Gmail and Google+, you may have noticed that a chat started in Gmail will carry over into Google+, and vice versa. The same isn't true for chats in Google Drive. Chats you start in Google Drive won't carry over into other Google products, and chats you start in other products won't appear in Google Drive."
{ via Google Drive Blog }