Launching an online community is really hard work. You need people who are going to spend days, perhaps weeks, personally inviting members to join your new community. (no copy and paste allowed, sorry).
It’s dreary work, but if you launched without a pre-launch strategy then you need to do this. Don’t rely on big hits on major sites to gain active members. Rely on what you control – personalised invites to a reasonable number of people.
This means at least one person spending hours at a time researching the right people to invite, writing them a personal invitations and welcoming them when they join.
If you do it right, you should gather a motivated group of people who want to be part of an online community.
But it’s still going to take days, weeks, maybe months to write 500 invites.