Click on Messages and click on Contacts. Here you can maintain your contacts list, update / add contacts and add contacts in bulk.
Click Messages > Contacts on Dashboard.
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.