I just remembered that the homepage of the forum is the mission statement. This is just a wild guess here but maybe the site doesn't get many new members because that page is a deterrent in some way. As either people don't see the link to the forum or maybe people don't have the patience to go through the extra page to make a profile.
Yes, that page needs updating, certainly. There is a security problem with it that will eventually be dealt with (it's not a problem for people that visit the page, rather a problem with a vulnerability that's being exploited to create spam pages using the domain name) and once that's done, one idea for it would be to have a "Latest Posts" area on it to show ongoing discussions.
Looks like a good plan, Recusant.
Unless one is entirely new to forums of the theism/sthrism type the basic "mission" is predictable and rules sre similar. Personally I don't bother with looking at them first, I take the "tone" of the forum from the actual posts - whether or not I have to join to get access. Getting "read only" access without actually joining is a bonus.
When I had a personal website I advertised the forums that I was on with a feed from a selection of the threads, indicating the breadth of matters covered, as a tempter.