When Office 365 backup is activated for the first time, our backup service needs to first retrieve the list of users from the company domain. Before the synchronization process is complete the user list in the backup portal only shows the global admin user. The synchronization process will start automatically and it can take a few hours or more to complete. You can also initiate synchronization manually by clicking the "Sync domain" button found under Actions on the Users page.
Why are there no users visible in the O365 backup portal?