select failed 3504 selected non-aggregate values must be part of the associated group - forum topic by jprumm

I'm having trouble with the following SQL and am receiving the error message in the subject line.  I know it has something to do with adding a GROUP BY because of the second item in my SELECT clause, but I can't figure out where it goes:

Select Distinct
  grp.GROUP_CUSTOMER_ID GrpNum
  ,max(b.Effective_Dt)
  ,grp.ExpDat
  ,c.PRODUCER_ID CURRENT_PRODUCER_ID
  ,c.CURRENT_PRODUCER_FULL_NM
  ,b.Agency_ID CURRENT_AGENCY_ID
  ,d.CURRENT_AGENCY_NM
  ,e.GROUP_COMMISSION_LEVEL CURRENT_GRP_COMMISSION_LVL
  ,e.INDIVIDUAL_COMMISSION_LEVEL CURRENT_INDV_COMMISSION_LVL
From
  (
    Select Distinct
      a.GROUP_CUSTOMER_ID
      ,Max(EXPIRATION_DT) ExpDat
    From
      PSOR_Temp.PRODUCER_X_GROUP a
    Group by
      1
  ) Grp
  Inner Join
  (
    Select Distinct
      a.GROUP_CUSTOMER_ID
      ,a.PRODUCER_ID
      ,a.EXPIRATION_DT ExpDat
    From
      PSOR_Temp.PRODUCER_X_GROUP a
  ) Prod
    on grp.GROUP_CUSTOMER_ID = Prod.GROUP_CUSTOMER_ID
      and grp.ExpDat = prod.ExpDat
  Inner join
  PSOR_Temp.PRODUCER c
    on Prod.PRODUCER_ID = c.PRODUCER_ID
  Inner Join
  PSOR_Temp.PRODUCER_X_AGENCY b
    on Prod.PRODUCER_ID = b.PRODUCER_ID
      and b.BUSINESS_TYPE Like '%group%'
      and prod.ExpDat between b.EFFECTIVE_DT and b.EXPIRATION_DT
  Inner Join
  PSOR_Temp.AGENCY d
    on b.AGENCY_ID = d.AGENCY_ID
  Inner Join
  PSOR_Temp.AGENCY_HISTORY e
    on d.AGENCY_ID = e.AGENCY_ID --and e.EXPIRATION_DT> date
  Inner Join
  (
    Select
      Agency_ID
      ,max(EXPIRATION_DT) EXPIRATION_DT
    From
      PSOR_TEMP.AGENCY_HISTORY
    Group by
      1
  ) f
    on e.AGENCY_ID = f.AGENCY_ID
      and e.EXPIRATION_DT = f.EXPIRATION_DT

Any help would be greatly appreciated.  Thanks in advance--