If a user tries to sign in to our Ontario GeoHub site, the sign-in window that pops up defaults to the AGOL organization that the Hub site is hosted from:

User try to sign in with their credentials from another organization and receive an error. I realize that they can click the "Sign in to your account on ArcGIS Online" link, but many users don't really understand the difference between different orgs and get frustrated. It would be better if the sign-in defaulted to arcgis.com