If you are setting up an account for the first time, the app asks for a user name and password that you can use to access the account. You enter the user name you wish to use, but if someone else already has an account using that user name, you will be told it is already in use and asked to enter a different one. Sometimes they even suggest a new one, often the same with a few numbers added to the end. Simply keep entering names until you get one that is accepted.
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If you are setting up an account for the first time, the app asks for a user name and password that you can use to access the account. You enter the user name you wish to use, but if someone else already has an account using that user name, you will be told it is already in use and asked to enter a different one. Sometimes they even suggest a new one, often the same with a few numbers added to the end. Simply keep entering names until you get one that is accepted.