Several years ago, we made a change that allowed an end-user to use their unique email address to log in. Simply, they are able to use this instead of their OpenAthens username. Because some duplicate email addresses existed in the system (for legitimate reasons), we had to make it opt-in. You may have noticed the checkbox in the account create wizard.
If your end-users will all have unique email addresses, then you can change this to be on by default for new accounts. People will then be able to sign in with their OpenAthens username or their email address. Or whichever they find easier to remember.
This setting is a per-organisation setting. This means that if you have sub-organisations, you have the flexibility to set a different default.
Access the setting in the admin site under Preferences > Organization.
For details on email address login and other settings such as the default expiry date, see the online documentation.