The first people to join your online community should be people that you know. If you personally know 50 people in your community’s topic field, you’re going to be off to a flying start.
If you don’t, then you should get to know 50 people in your community’s field before you launch your online community. See the 3 month pre-community strategy for help on this.
The second group of members to join your online community should be people that your friends know. You need to motivate them to do this.
If you do this right, your community should grow from 20 to 50 to 100 over the first month or two. That’s a good number of engaged members.