I run a pretty active community of about 650 in the local news publishing space. Start when you have a handful of smart people with a common agenda (we original

I run a pretty active community of about 650 in the local news publishing space. Start when you have a handful of smart people with a common agenda (we originally called it "the thinktank"). Then organize them using software that stays in their day to day workflow like an email list or social media group. Despite its shortcomings, I used Facebook groups and it works kind of well. Then link to, market, and promote your group wherever appropriate. Ours is still approval-based to keep the quality high and the users engaged.