Communties succeed when they reach a critical mass of activity, not a critical mass of members.
If you think you need lots of members to get activity, you're wrong. You need lots of activity to get members.
The goal then is to reach that critical mass of activity amongst the members you have. It might be 5000 interactions when you have 100 members and 25000 interactions when you have 500.
Naturally, you need a few members to get started. Members need others to talk to. But this number if far, far, far, lower than we think. Any community that begins with a big promotion push fails. It's the communities that start small, reach a critical mass of activity per member, then keep growing at that level of activity which succeed.
This means, right now, you need to focus on starting discussions and measuring the outcomes. Find the themes that work best and use them frequently. Write content about the community, and measure the types of content that work best. Ruthlessly measure the level of activity per member and adapt as you progress. Organize events for members, engage in hundreds of personal interactions with members, provide them opportunities to get more involved in running the community or stand out amongst the community.
There isn't a critical mass of members, it doesn't exist. Luckily, a critical mass of activity is very much possible – and not as hard to achieve as you might imagine.