Private communities on Crowdicity are not listed within the app. Instead, users must enter a "Short Name" to find the community. This must be configured in your community settings, or users will be unable to use the app with your community. However, it will still be available by visiting the community's URL via a mobile browser.
You can find the field for this under Community Settings -> Access & Privacy
It may take up to 5 minutes after adding the short name before the community is available within the app.