Ok, so I just joined another group that is positioning itself to be the Global chat replacement, and I was looking at the pending invites and I wasn't in the list (I just joined because I saw clare joined), and it occurs to me that if you only see chat channels for groups that you belong to and hear about them via your network...well, that's not really very public, is it? The serendipity (what I like to call it) of Global chat can't be nurtured that way, can it?
At the very least, in the chat UI, it would be good if a player could see which channel has the most players in it at a given time, so they could go join the group and check it out (Invite-only groups being the exception, those would not show up). It's kind of a bee-yatch to have to flip out to the groups page, find the group and then join it too, so maybe you could get the option to join a group quickly via the chat UI.