I moderate one of the earliest discussion boards on eBay called Sterling Silver Lovers, with over 1000 members. I opened it the day that the discussion board feature came out. I was a Powerseller of antique sterling silver at the time, full time, under the name, thebidwatcher. I do not work at that job now but I still moderate that board, dropping in now and then.
The first and most important thing you must decide is why you want to do such a thing. I wanted to draw like-minded interest eBay members to my auctions, simple as that. By sharing info and being top of the page every time, my name, link and score was there. Period.
Secondly, you must determine if the people who members who join are there to make trouble for you or contribute. By make trouble I mean surf for suckers, spam accounts, play alpha know it alls or general PITA's. At one point there were folks crashing into the eBay accounts, so the group had to go from public to private, and I had to decide who got in (whoo hoo, power!). I just denied anyone who had an account less than 30 days old.
One more thing I suggest is that you figure out how to tack repeat information at the top of the board, like reference material or board rules, if you have any. That way new members or forgetful ones don't have to dig for answers or ask over and over.
Hope you find this helpful and happy Foruming!