Before proceeding to enforcing SSO, you should have already:
Added and verified the expected domains to your Organization
Invited all your existing users as managed users into your Organization
To enable SSO for your users, you can select one or more verified domains to enforce SSO for them.
You must enforce SSO only if the test SSO connection is successful. If you add domains to enforce while the SSO connection is not working, then all your users from those domains will face login failures.
Also note that when SSO is enforced for any user, they will only be allowed to log in using SSO. They will not be able to use their existing password to log in. So you should inform the users in your Twilio Organization before enforcing SSO for them.
Here's what will happen once you enforce SSO on a domain -
If you want to add or remove domains from an SSO Profile, you can do so by editing the SSO Profile.