Why bother with the temp table? Why not just imbed the select into your where clause?

SELECT

SO.SalesPersonID, C.FirstName + ' ' + C.LastName AS SalesPerson

, COUNT(*) AS Orders

, SUM(SO.SubTotal) AS TotalAmount

FROM Sales.SalesOrderHeader SO

INNER JOIN HumanResources.Employee E ON

SO.SalesPersonID = E.EmployeeID

INNER JOIN Person.Contact C ON

E.ContactID = C.ContactID

WHERE SO.OrderDate BETWEEN @StartDate And @EndDate

-- The dynamic IN predicate.

AND SO.SalesPersonID IN (SELECT CONVERT(INT, Item) AS SalesPersonID

FROM dbo.DelimitedSplit8K(@SalesPeople, ','))

GROUP BY

SO.SalesPersonID, C.FirstName, C.LastName