What's going on with the login? Where are usernames?

What's going on with the login? Where are usernames?

I have 75 users created with username and password.  Suddenly I am unable to create an account with a username, apparently I MUST use the email address.

This has created TREMENDOUS confusion.  What happened to all the accounts created with usernames?  How do they log in?

Do I have to tell everyone to use the email address to login?

Also when I try logging in with the email address, Zoho creates its own username which has nothing to do with the email address.  What in the world is going on?????????

Please help.  This has screwed up my enter business.