When a team member tries to log in to a LaunchDarkly account which has SSO enabled, they should be redirected to login via your Identity Provider. If a team member is prompted for a password, this indicates that an account has not yet been provisioned for that team member.
LaunchDarkly does not support automatic provisioning of new team member accounts. To add new team members, the initial login must be done via a LaunchDarkly application set up in your Identity Provider. Once that step has been taken, the account will be created. On subsequent logins, the team member will be redirected to your IdP when attempting to log in via the LaunchDarkly website.
If you get an error saying that you can't join because the email is part of an existing account, then you should follow the steps in this knowledge base article and then try logging in through your IDP again: https://support.launchdarkly.com/hc/en-us/articles/115002410848
Read more about user provisioning in our documentation here.