Clearly communicate the rationale or goal of the project within your community (e.g. Help us identify as many species as possible in our local neighbourhood and contribute to biodiversity research).
Be willing to put in the work to promote the opportunity within your community. A little passion goes a long way.
Recruit additional admins, managers or curators to your project to help with identifications and community-building. Admins can be added on the Edit Project page for a Collection or Umbrella project. In your Project settings you can add "Members" at the very bottom of the settings page and make someone a manager or curator on the project.
Run contests or organise meet-ups for your local community members, friends or family. Themes like the "observation of the month" or year-long challenges encourage interaction and friendly competition among project members.
Acknowledge community members for their findings, however significant or insignificant. Getting a comment on your observation by the group administrator can help encourage participation. To post a comment click on an observation in the project.
Keep track of the progress of the project. It is a powerful tool for sharing good news stories on the accomplishments being made by participants in your project.