Plus! does not automatically open contacts into specific tabs by group yet, however i like this idea and i can see its uses so Patchou may see this also and add it in a later version
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You can have several Tabbed conversations windows open though. This information is shown in the Changelog available here:
http://www.msgplus.net/changelog.php
But, here is the one you'd like specificially:
Shortcut keyboard F9 groups all chats and Ctrl+F9 adds the current chat to the group. Shortcut keyboard F8 ungroups all chats and Ctrl+F8 ungroups the current chat from the group.