Even though it is advised to add contacts using their professional email address, it is also possible to add a contact using a private email address.
However, people cannot register themselves using a private email address – they can only join on invitation.
When responding to the invitation, the invitee will be prompted to enter a professional email address. If he/she does not, he/she will become a user with limited rights.
If this contact is one of your co-workers, you can also ask the company admin to add them to the company account. This way, they can get an account without limitations and they can be added to the overview of co-workers.