1. Go to Marketing > SMS > Contacts, tab Lists, click New List, enter a Name and Save.
2. Open the list (click its name). The header shows "N members" and the buttons Add Users, Add Contacts, Add CSV and Edit List.
3. Add Users opens a searchable table of customers (ID, Name, Phone); tick the ones you want and click Add Selected. Add Contacts does the same for your standalone contacts.
4. Members appear in the table with Name, Contact and Type (User or Contact).

A person is only added once even if selected again. Members' names link to the customer record or contact edit page.