If you want to direct your members to sign up without being invited or added by an administrator, enable self-signup.
You can decide whether you want to allow members so sign up on their own by enabling self-signup in the System Settings portion of your control panel. When toggled on, 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. Admin's can see the members who need approval in the Members section of the admin control panel.
It's a good idea to assign members to groups at the same time that they are approved.
Best Practice Alert: Does your organization already have a website? Include a prompt for folks to download your app.
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.