Are there any known issues with it? I just updated on my Mac and don't appear to be able to group tabs - dragging one on top of another just reorders them
If the invitee turns out to be a spammer, or posts a bunch of hate speech etc, the inviter may be held responsible for the invitee's actions. This isn't a hypothetical, it's explicitly explained in on the invite form, and I've seen cases where people were banned because they invited people who then acted spammy.
I'm not against the invite-only system, having been a forum moderator for several years, but I'm happy to just be a consumer/observer of the conversation rather than a participant.
It also isn't intrinsically linked to the VC scene like HN, which may or may not be a plus/minus, depending on your views.