HierarchyID: Get all descendants for a list of parents
One way to accomplish this is:

USE tempdb;
CREATE TABLE dbo.PersonHierarchy
(
    PersonHierarchyID HierarchyID
);

INSERT INTO dbo.PersonHierarchy VALUES ('/1/');
INSERT INTO dbo.PersonHierarchy VALUES ('/1/1/');
INSERT INTO dbo.PersonHierarchy VALUES ('/1/2/');
INSERT INTO dbo.PersonHierarchy VALUES ('/2/');
INSERT INTO dbo.PersonHierarchy VALUES ('/2/1/');

SELECT *
FROM dbo.PersonHierarchy;

DECLARE @T TABLE 
(
    PersonHierarchyID HierarchyID
);

INSERT INTO @T
VALUES ('/1/'), ('/2/');

WITH parent AS (
    SELECT p.PersonHierarchyID 
    FROM dbo.PersonHierarchy p
        INNER JOIN @T t ON p.PersonHierarchyID = t.PersonHierarchyID
)
SELECT ph.PersonHierarchyID.ToString(), parent.PersonHierarchyID.ToString() /* select only the necessary columns, do NOT use SELECT *  */
FROM dbo.PersonHierarchy ph
    INNER JOIN parent ON ph.PersonHierarchyID.IsDescendantOf(parent.PersonHierarchyID) = 1
ORDER BY ph.PersonHierarchyID; /* always include an ORDER BY */