You can decide whether you want to allow members so sign up via the application or whether you want them to require an invite by jumping into System Settings. 

When toggled on (seen in blue), members will be able to sign up directly from the app without an invite. Members who sign up via the app will need to be approved by an admin before receiving access.

Note: If you allow members to sign up via the application, anyone who downloads the app or has the URL will be able to request to sign up for your group.

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