Private apps require users to sign in (no'create account' option). All private app users must be invited to the app by being added to a group.
Invited users are sent an account activation code and download link.
Public Users /Signed Up Users
Users who already have a platform account can sign in to public apps using their existing credentials.
If you have a public app users can sign up to your app without being added or invited to a group.
See who signed up to your public app
To see which users have signed up, open Groups and then select the "Users" tab from the top menu. This will show a list of all signed up users and their related details. You can use the export option to extract the user list.
Any user who signs up will not belong to a group (unless later invited to a group). The user will, therefore, show as belonging to "No Group".