The only way for the user to join existing Company is to select existing company from a drop-down box during the registration process. On a first step you will need to select “Company” as account type, and then proceed with selecting a company from a drop-down list:
After successful registration, the Admin of selected Company will have to confirm a user. For that, he will need to change user status to “Confirmed” on Company Team screen.
Once the Admin approves a user, the user access to the company’s account will be granted and user will be able to log in.