You cannot add a colleague as a contact using the email address they used to sign up to Karbon with.  

The risk with adding a staff member as a contact is that once you have shared your communication with that staff member, all other staff members will be able to see every email sent to or from the staff member. 

While this is great for contacts, often internal communications are sensitive (eg. HR, pay, holidays) and sharing all those communications for a staff member can be undesirable.