When you have an Office 365 subscription you get access to Lync software.
If you install Lync and you want to add people to your contacts you have to manually add each user.
Currently there is no official solution to achieve an automation for this.
There is however, a workaround:
Log in to the Exchange Admin Center.
Go to ‘groups’ and create a new Distribution group.
Add every user to this group and click save.
In the Lync software you can search for the dist group you just created.
Right click the group and choose ‘Add to contact list’
You now have every user in that group as a contact!