It sounded like you'd already identified a few of them. Here's one we tried, though:
I formed a group with player A in one zone.
Zoned to another zone and invited players B and C while player A stayed behind.
Players B and C could speak in group (a huge improvement), but player A's chat would sometimes come through and sometimes not.
After player B zoned (we're now spread across three different zones), player B could see his chat go to group, but the group couldn't see it.
Again, though, it sounds like you already have a pretty good idea of some of the problems, and I'm pretty sure this falls within the realm of what you'd described above. ("Players invited to a group wont learn members that are out of the zone and the members out of the zone wont get member updates.")
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