The Google Talk server does not support Multi-User Chat. If you want to chat with multiple people at the same time you need to tell Coccinella the address of a server that does support Multi-User Chat. You can do it like this:
Log in to your Google Talk account using Coccinella
Press Ctrl+D (or Command+D on Mac OS X) to open the Discover Server dialog
Enter a server that supports Multi-User Chat or select one from the dropdown list (e.g. im.flosoft.biz)
Check the option Discover permanently to automatically discover this server each time Coccinella is started
The Google Talk server does
The Google Talk server does not support Multi-User Chat. If you want to chat with multiple people at the same time you need to tell Coccinella the address of a server that does support Multi-User Chat. You can do it like this: