First Inside CostOS you have to add the users inside CostOS.In that you want to. So, in the top right corner of the screen, click on Usermanagement.
It is needed to add usernames that are the same with the yours domain usernames. Then, you have to add a password. (In a future release this won't be required, so it is recommended You don't need to add a strong password )on our latest releases.
Then you could add as many users you like to.
Finally, when you try to login :
You just have to click on SSO enable SSO login and then click the button login.