You can add contacts to your network profile in two ways:

A. Sending a contact request.
You can add contacts by clicking on invite contacts in the drop down menu under Contacts in the menu bar of your network profile. Then you can enter the email address of the contact you want to add, it is optional for you to include an accompanying message. Then click on send invitation.
In the same drop down menu as above you can also choose Search contacts. Then you fill in the name of the person you want to add to your contacts list and click on search. Once the right person is found click on Add as contact.

B. Accepting a contact request.
As soon as someone sends you a contact request you will be able to see this on your menu bar. By clicking on the symbol with two people you can accept or decline the request.