Like others, I think the obligation on the host is to be clear on the nature of the event. In the "Girls' Only" thread...the problem is not that she is obligated to tell her friends that her boyfriend is coming, it's that she advertized the event wrong. It's not Girls only. Perhaps she was trying to communicate that their SOs were not invited as well, but she did a poor job of communicating her event and it lead to confusion and apparently some bad feelings.
The same with Kate. She's not obligated to tell Jill that Beth and Sarah are invited, but she is obligated to tell Jill that she's being invited to a group event vs a one-on-one thing.
And I think I understand why there's some heartburn about this. On one hand, we say that the host is obligated to be clear on what the event is so that the invitee can make an informed decision on whether or not they would like to attend. But at the same time, we say that the host isn't obligated to reveal the full guest list as it's rude for one person to make a decision on attending on anything other than their enjoyment of the hosts' company and the event at hand. Considering that the guest list does add to or take away from the event dynamic, I understand why some people would like to know.
But, in general, I still think that it's best not to ask who else will be there before making a decision. Not only because it can give the impression that you (general) may not like to spend time with the host unless someone "better" is also in attendance, but in the case where you just don't like someone, it can put the host in an awkward spot and basically asks them to constantly choose between you and the person you don't like.
So, if you really really can't stand someone that you know your intended host hangs out with, then it may be best to either not accept offers to group events or get to the point where you can tolerate their presence (at least at certain larger events/settings) and accept those invites.