If a new Contact (person) is intending to create a new login to place their first Order Online and one of their colleagues already has a login, then they should instead ask you to create their login for them.
The reason why is because, if that person creates a login Online, it will create a duplicate Customer (organization) in your Back Office system, which may affect your ability to Invoice that Customer.
By creating the Contact within the existing Customer profile in the Back Office you will prevent duplication from occurring.
Once you have created the Contact login (with their email address), the Contact can use the 'create a password' function on the Online Ordering site to help them proceed towards logging in.
Transferring a Contact
It is possible to Transfer a Contact who is in a duplicate Customer. For information on how to do this, see Transfer a Contact to a Different Customer.