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)

;