Employees can't do first login

Hello everybody, I'm having problems with teams for a customer. An employee has registered with teams with his company email address (free of charge as far as I know). Now other employees come and want to register with their company email address, but they get the error message "******** was not added to the directory of your organization" ... I assume the first employee to sign in to Teams is the administrator. This employee is the managing director and won't let me go to his team. He is also currently invited to another organization with his teams account. What can I do???