The third task is straightforward: Create a CargoX account for your colleague (you can only add another user, if your account has permissions to do so).
Your company's CargoX account can have many users under it.
Due to security, users can not register their CargoX accounts directly and then nest themselves under an existing company account. The right procedure is for existing users belonging to a company, to create new users from the platform itself.
HINT: Manage | Users.
When setting up a new users's account you have to set their permissions. Users can have various access levels - according to their job title and responsibilities.