What is my user ID and why do I need it?
Your user ID is the new email address that you create when you sign up for Microsoft Office 365. You use this user ID, along with the password that you also create, every time you sign in to Office 365. An initial Office 365 user ID looks similar to the following: email@example.com.
You have to use a unique user name for your user ID, and for other users you create for your Office 365 account. The domain part of your user ID—the part to the right of the @ symbol—is either the “.onmicrosoft.com” domain that you chose when you signed up for Office 365 or another domain—for example, your organization’s custom domain—that you’ve added to Office 365.
For example, if you chose fourthcoffee.onmicrosoft.com as a domain name when you created your Office 365 account, and you chose a user name of colinw, your Office 365 user ID would be firstname.lastname@example.org.
If you want your user ID and Office 365 email address to use your custom domain—as in, for example, email@example.com—instead of the initial .onmicrosoft.com domain, see Options for adding your domain to Office 365.
When you sign up for Office 365, you also need to provide an existing email address as your alternate contact email address. After you complete the signup process, you’ll get an email message at your existing email address that includes your user ID and the URL of the page where you will sign in to Office 365. If you forget your user ID, or if you don’t remember the URL of the Office 365 portal, look at the introductory email message to find this information.
You’ll create a new user ID for each user account that you add to your subscription. Office 365 generates a temporary password that you can give to new users, along with each user’s user ID, so they can sign in to Office 365.
The alternate email address you are asked to provide during the signup process is used for important communications, such as administrator password resets. Information about service and billing, and promotional information that you choose to receive, will also be sent to this email address. If you are an administrator, you can change this email address on the My Profile page.