1. Login to the Web client “https://client-hostname:9443/vsphere-client” with administrator@vsphere.local using the password of what ever you configured SSO password as you can only configure SSO using the Web Client*
2. Navigate to vCenter Servers > Manage > Permissions, click on the + to add a user
3. Now add your Domain user or group that you would like to have access to vCenter along with the permissions required and click on OK.
4. Try and login with the user you just configured and you should now have access to the Web client or the Windows vSphere UI.