hard to do with HTTrack.
I've done it with wGET, because with wGET you can specify "relative links" and
it'll go download the entire BBS without going off onto other sites. However
it may not be perfect.
wGET has problems because of Robots.txt, which may kill it before it gets
going. Depends on the site.
nav to the site with your browser keep it onscreen
go into WinHTTrack's options and set the BrowserID to be the same as your open
browser. You may need cookies too.
set the filters on the options to only take the membership page urls. like: -*
set depth to like 100 or so.
set your Limits to maybe 10 megs or 10 minutes.
then let it takeoff and see what it finds.