can't easily select which members of a group you want to send the invitation to.

You have to look at why Facebook exists for that one. They want people to be inundated. I have an account purely for family and after a day of no new posts I get three classes of assinine adverts 1) "comment on this post from 10 years ago" 2) "add friends and you will see more posts" 3) "here are people you may know, go on, add them"

And yet I can't see posts from the previous day. Call it filtering, or censorship - but lets face it Fakebook is lonely. And a lot FAKE.

FWIW your Fakebook group needs to spell out the correct way to enter an event in the "about" section at least. As admin, can't you do that?
If Julie Smith knows how to do that she should be communicating the knowledge. And the acceptance procedure should be listing the no-nos and requesting you acknowledge reading them before admitting you to the group. Some other forums do exactly that.

Post Code - a lot of "organisers" know where their event is, and don't spot the need. BTW you can get the post code of village halls (eg) by using the nearest house or one of the many "Halls for Hire" websites. I even downloaded the OS free pack, and run it in Excel (macros) so I can generate OSGR off line - or even PC from OSGR if I can see the OSGR. Otherwise I use Streetmap.co.uk

All my entries for mister.red & dance.mister.red have both PC & OSGR. In fact http://map.mister.red uses OSGR to place the dots.

Post Codes are notoriously imprecise. They are designated for the volume of mail. Which, in rural areas, can be a mile or more across.