How so? There are no exemption for NSC/NRA in primary legislation. Either the Police turn a blind eye, or they just carefully manipulate the line between clubs and private ranges to get the benefits of having people as members of a club, but to avoid the pitfalls of having to abide by a club's HO Approval conditions.
You can't just turn up to an NRA Open day - you must sign up in advance, so arguably, your name will be personally known to the person handling the registrations.
I don't know how they do it, but they are the only people who can have open days to members of the public. The one I went on not more than 4 years ago didn't even need preregistration, you brought your form along on the day.

As for 'known' I guess as with all laws it needs a test case. How about Crown V Hemmers for taking a close mate he met in the pub 10 minutes before going to a a club guest day

You can be friends with someone for years and not really know them.