That message generally indicates the account has been deleted.
The following information will tell you how to find out if it was and reasons an account can be deleted.
Reasons Yahoo deactivates or deletes an account | Yahoo Help - SLN3057
Confirm if your account has been deleted using the Sign-in Helper. If your account doesn't exist anymore, you'll get a notification page saying it has been deactivated or you can't reset your password online.
Depending on your account's activity or a request originating from your account, it's possible for your account to be deleted from Yahoo. Once your account has been deleted, it and all its information will no longer be available to you.
You can always sign up for a new Yahoo account, just keep in mind that your old Yahoo ID may not be available for you to use.
Actions that may result in an account being deleted
Based on what the Yahoo! admins have been posting, a gmail address should work just fine.
Try it, it can always be changed if it doesn't work for some reason.
I have 2 yahoo accounts, one with my gmail address.
They all agree until someone gets an eye poked out LOL.
If you are intent on doing it then post a list of his predraft rankings and then stick to it no exceptions.
And yes you will need two browsers as stated.
If the missing player wants his team drafted by another person, they would have to provide their ID and password to that player.
The player would then need to open 2 separate browser(like Chrome and Firefox) on 1 device or use 2 separate devices to draft his own team and the team they are covering for.
The person drafting for both teams is asking for nothing but trouble.