Here is the common approach to growing a community. Initiate a big promotional drive. Get coverage in major blogs, host a major competition and attract 50,000 visitors to the platform. After which perhaps 500 join the community and 50 are still active after 3 months.
In approach 1 the bulk of the effort is undertaken before members join the community. Compare this with approach 2. Here the community manager wants to grow the community. They work to increase referrals from existing members. They get 1,000 members to visit the site and a high number to join, perhaps 80%. They work hard to keep these members and ensure they stay engaged. After 3 months, 500 are still active.
These numbers aren't uncommon for either approaches.
If you want to grow your community focus your efforts after members have joined, not before.
You might think it's the chicken and the egg. If no-one visits, you can't keep them engaged. However, if you take care of the members you have, they'll bring in more members. Most successful communities have never undertaken any promotional activities at all.