If you want separate columns for first name and last name:
SELECT
ci.ClientID,
Person.query('.') AS Person,
Person.value('FirstName[1]','varchar(20)') AS First_Name,
Person.value('LastName[1]','varchar(20)') AS Last_Name
FROM
ClientInfo ci
CROSS APPLY
Info_untyped.nodes('/People/Person') AS People(Person)
;