Give them a signup bonus, e.g. signup & get free ebook.
Do content marketing: basically spamming your blog posts as links everywhere. The blog acts as a lead in: if people are interested in that topic they may want to participate in discussion about it.
Some kind of referral bonus system (works better when already have a core of initial users).
Doing giveaways: e.g. I'll choose a post at random and send them a gift card, which encourages engagement.
Focusing on specific areas where there's no other good place to discuss a niche topic.
Do content marketing: basically spamming your blog posts as links everywhere. The blog acts as a lead in: if people are interested in that topic they may want to participate in discussion about it.
Some kind of referral bonus system (works better when already have a core of initial users).
Doing giveaways: e.g. I'll choose a post at random and send them a gift card, which encourages engagement.
Focusing on specific areas where there's no other good place to discuss a niche topic.