Here you can maintain your contacts list, update/add contacts and add contacts in bulk.
Navigate to Messaging tab. Click the name of the inbox and select Contact List from the options.
Uploading a Contact List
Click on the Upload CSV. Select a File to Upload (if you don't have a CSV, you can download a template).
Note: be sure your CSV has the phone column and a name column or a firstname and lastname column.
Preview your contact list, then click Confirm. For more information, click here.