I'm not a member of Facebook so I don't know what people see when they visit the Mudcat group, perhaps using a mobile app. But at the URL you posted - https://www.facebook.com/groups/mudcat/ - I see an 'about' text:

This page is for announcements, as an annex to the official Mudcat.org site that has been around since the mid-1990s. It was set up by and is owned by Max Spiegel, who has created a wonderful site… More

Do members see that every time they visit or post?

It reminded me of a usenet group charter, and that some groups used to post that monthly to remind people of the group's rules. So maybe someone could post a monthly charter to the group reminding people of the website and encouraging people to archive non-ephemera here. The announcement could probably be automated.

BTW, it says the group is public and that 'Anyone can [see] what [members] post.' But as a non-member I can't read people's comments - e.g the three comments under that Raglan Road post. Do those comments ever contain discussion about, say, the origins of a song?