Microsoft To-Do went into "Preview" in April 2017, though this period has since ended and the product is now what is considered "generally" or "commercially" available to Office 365 customers. As such, Microsoft To-Do is turned on for customers by default and access is managed through per-user licensing.
Users on "preview" versions of the Microsoft To-Do app (v1.18 or older) are still subject to their admin's preview preferences, which are saved in the Office 365 Admin Center via an opt-in toggle.
If you wish to update this setting for your organization, open the Office 365 portal and click on the "Admin" tile.
Please remember that in addition to this organization-wide access control, there are now service plans in Office 365 suites that act as user-level licenses and can be used to manage individual access to To-Do. To learn more, please read For Admins: Assign Microsoft To-Do licenses.