Group chats tend to be devoted either to topics or to teams. Topic-driven group chats are typically open so that anyone can drop in to ask a question.
Funny they should mention this since Google doesn't really have great support for topic-based chats. Chat rooms aren't discoverable (so you can only join a topic by being invited to it) and there aren't good notification controls so that you can have passive and non-spammy relationships with topics. Add to that that channel changing isn't frictionless (can't have unread channels at top of list, can't easily keyboard-shortcut open them, and channel changing is _slooow_). Adding yet more, Chat's threads are awkward, to say the least.
