This issue has been resolved at Dec. 26th, 4:30 pm JST.
We sincerely apologize for any inconvenience caused.
Currently Dec 25th 2018 11:00 a.m. JST, SkyDesk CRM causes a failure if you invite a new user.
If you add a new user on CRM, invitation mail will be sent for the user.
However, the user cannot "Accept" the mail.
We sincerely apologize for any inconvinience caused.