Step 1: Login into portal.office365.com
Sign in with your Login ID and Password.
Step 2: When entering account click on Admin as shown into the picture.
Step 3: In the Admin Panel click on Azure Active Directory.
Step 4: Select Users then click on the New Guest User.
Step 5: In the New user page, select Invite user and then add the guest user's information, after that click on Invite button.
Now we can see that we successfully add a guest user.