What is my user ID and why do I need it for Office 365 for business?
Your user ID is assigned to you by your work or school. It looks like firstname.lastname@example.org, email@example.com, or firstname.lastname@example.org. This work or school account is the email address and account that you use to sign in to Office 365 for business for work or school.
Your work or school account is different from your Microsoft account. Your Microsoft account is the one that you use for personal services like Xbox Live, Outlook.com, Windows 8, Windows Phone, and more. Check out this video, What is a Microsoft account? for more details.
You can't use your Microsoft account to sign in to Office 365 for business for work or school, unless someone in your organization shared a specific document with you via your Microsoft account. Then you could access that document using your Microsoft account.
When you get your work or school account for the first time, you'll also get a temporary password to sign in with. When you sign in, you'll be asked to create your own password. Use it with your user ID every time you sign in to Office 365 for business. For more information about changing your password, including how to create a strong password, see Change your password.
Office 365 for business admins
The following information is for Office 365 for business admins only.
Create new user accounts
You’ll create a new user ID, with a unique user name, for each user account that you add to your subscription. The domain part of the user ID is either the onmicrosoft.com domain that you chose when you signed up for Office 365 for business or another domain—for example, your organization’s custom domain—that you’ve added to Office 365 for business.
For example, if you chose fourthcoffee.onmicrosoft.com as a domain name when you created your account, and you’re creating a user account with the user name colin, the user ID would be email@example.com.
If you want to use your custom domain for user IDs and email addresses, see Verify your domain in Office 365.
Set up and use an alternate email address
When you sign up for Office 365 for business, you also need to provide an alternate email address that’s not connected to Office 365 for business. This email address is used for important notifications, such as resetting your admin password, so you must be able to access the email account if you don’t have access to Office 365 for business. After you complete the sign-up process, you’ll get an introductory email at your alternate email address that includes your user ID and the URL of the page where you will sign in to Office 365 for business. If you forget your user ID, or if you don’t remember the Office 365 for business URL, look at the introductory email for this information.