Your Contacts is your address book. It is a list of all the people you have added and/or communicated with up to this point. When you add a new contact, their identity is automatically shown on this list. See how to add new contacts to Your Contacts.
Your Contacts can be accessed at any time from the left side menu by clicking Contacts.
From here you can drag and drop people into Channels. See how to add people to Channels.