ArcGIS Hub now enables community members (people who do not belong to your ArcGIS Online organization) to authenticate their accounts using an identity provider. This means that instead of signing in with Google, Facebook, or another email account, community members can use a third-party provider to sign in to ArcGIS Hub. For example, some people already have a login provided to them by their government or organization. Now, they can use this login to access their community's ArcGIS Hub. There's no need for a second login.
To enable SAML for your ArcGIS Hub, you must have access to the ArcGIS Hub community organization. Note that this organization is separate from your primary ArcGIS Online organization and therefore, requires a separate set of login credentials. Here's how to set up identity provider logins for your community organization.
The community org sign in/sign up settings will now appear on all sign in screens across ArcGIS Online. To create a community account, visit your community's ArcGIS Hub site and click Sign In at the top right of the site. Or, sign in at hub.arcgis.com.
For more information, read the ArcGIS Online documentation on how to Set up enterprise logins.