Step 1: Login into portal.office365.com


Sign in with your Login ID and Password.


Continue…

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.