How to Invite External user on your VSTS
One of my colleague was want to add external user in VSTS. but while he is trying to add external user and typing user name then only organization users are appearing in the drop down.
Resolution
Login to Azure Active Portal with admin right or contact you AD administrator
Go to Azure Active Directory
Select Users Menu
Now click on New Guest User and save it.
Now login again to your VSTS and try to add users
