You can add a single contact to an SMS list by following the below steps.
1. Navigate from the home menu to Contacts.
2. Go to View Contacts.
3. Click Add Contacts.
4. Then Single Contact.
5. Enter the Phone Number.
6. Select a list to subscribe a contact to from the drop-down menu.
7. Click Continue.
Next, enter any additional information (system fields or custom fields) about the contact, such as:
First and Last Name
8. After you have entered the applicable information, click Save.