When a user tries to log in to the Bitrix24 account, he/she may get the error message:
This error may occur if the user has already logged in to the Bitrix24 Network account that is not connected with the selected Bitrix24 Account.
There are two possible reasons for that.
This user has never existed on the selected Bitrix24 Account
In this case, an administrator needs to invite the user to this Bitrix24 Account.
This user exists on the selected Bitrix24 Account, but uses the wrong login
In this case, it means that the user has used a different email address as a login or has used a social network account to log in.
The Bitrix24 Account administrator can check what login does this user have and send this information to the user.