First you have to add the users inside CostOS.
In the top right corner click on Usermanagement
Then you
Usermanagement.
It is needed to add usernames that are the same with the domain usernames.
Then, you have to add a password. (In a future release this won't be required, so it is recommended to add a strong password).
Then you could add as many users you like to.
You just have to click on SSO login.