From your Homepage, you have two methods to add a new contact.

Méthod 1 : add a new contact from the left side menu

You can add a new contact from the search bar of the left menu, by positioning on the "Messages" tab and by using the "+" button:

You are then asked to enter either the email or the ID of the contact to invite, and an optional message.

Méthod 2 : create a new contact from the navigation bar

You can add a new contact by clicking the button on the navigation bar at the very top of your homepage. A drop-down list proposes to you to create a message for a contact, a new notebook :

You are then asked to enter either the email or the ID of the contact to invite, and an optional message.