Unsuccessful communities focus on getting members to register.
They spend their time promoting the community. They ensure people get through the registration process. Some might offer incentives, others spam their mailing lists. A few even buy advertising.
Successful communities put the effort in after the registration process.
They fight for every member. They introduce members to others. They ensure members make a contribution on their first visit. They initiate and support interesting events/activities for members. They start discussions and prompt people to respond to them. They create content about what members are doing. They take the time to build genuine reationships with members.
They're successful because word spreads. People hear about and join great communities. Media channels pay attention to what successful communities are doing.
All the effort you invest in outbound promotion, is usually better invested in doing exciting activities within the community.