So, you already have a Google Account?
If “Yes”, then see these tips about changing your User Name:
Google Help does not provide clear information on how to add another email address because it mixes up information about “User Names” and email addresses.
The tips page above includes information about adding email addresses.
If it is all as clear as mud please do not blame me – it is just the way that Google Accounts and Google Groups have been set up by Google.