invalid in the select list because it is not contained in either an aggregate function

Question

I can't figure out how to rewrite this sql statement to avoid the error below

Msg 8120, Level 16, State 1, Line 26 Column 'StaffMember.StaffMemberId' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

SQL Statement:

SELECT (
        SELECT [dbo].[udf_ReadStaffMemberBasic](SM.StaffMemberId)
        FOR XML PATH('StaffMemberSummary'),
            ROOT('Items'),
            TYPE
        ),
    COUNT(SM.StaffMemberId) AS TotalResults,
    CEILING(CAST(COUNT(SM.StaffMemberId) AS DECIMAL) / @PageSize) AS TotalPages
FROM StaffMember SM
INNER JOIN StaffMemberToAdditionalRole AR
    ON SM.StaffMemberId = AR.StaffMemberId
WHERE (
        SM.Firstname LIKE '%' + @Name + '%'
        OR SM.Surname LIKE '%' + @Name + '%'
        )
    AND (
        @FloorId IS NULL
        OR SM.FloorId = @FloorId
        )
    AND (
        @DirectorateId IS NULL
        OR SM.DirectorateId = @DirectorateId
        )
    AND (
        @AdditionalRoleId IS NULL
        OR AR.AdditionalRoleId = @AdditionalRoleId
        )
ORDER BY SM.StaffMemberId OFFSET @Page * @PageSize ROWS

FETCH NEXT @PageSize ROWS ONLY

What I'm trying to do is to return a list of Staff members from a function and pagination data all as XML.