Yes, it is possible.
From the "users" screen, select "Add user", then choose "Without email address".
Related:
Invite Users without Email Addresses (Web Browser)
Notes on not registering an E-Mail Address
- Users will not be able to receive E-mail notifications, including emails containing the initial Login information for the Teachme series.
The Administrator should inform the account users of their Account name and Password by some other means. - If a user forgets their login Password, they will not be able to change or Reset it themselves and will need to contact the Administrator.
- Users will not be able to receive any notification emails from the Teachme series.
- Users cannot become an Administrator, such as an account Administrator or Folder Administrator.
If you wish to receive notifications without registering an E-Mail Address
For products that have a notification feature in the mobile app, please consider using "Information on Updates" or "Push Notifications".
Please note, however, that the information delivered via the mobile app notifications is limited.
Reference: [Admin] No-Email User Password Reset (Web Browser)
https://teachme.jp/8/manuals/51167148