SQL & PL/SQL :: Adding Inner-query Select To Group By?

I have this query

select EAG.AUDIT_NUMBER Audit_Nbr,
EAG.AUDITEE_NAME Grantee_Name ,
EAG.EIN Grantee_EIN_IRS,
EAG.AUDIT_ISSUE_DATE Audit_Issue_Date,
MAX(AUDT.derive_audit_progress_status(EAG.SYS_AUDIT_ID )) Audit_Clesed_Date,
EAG.OIG_DUE_DATE Six_Month_Due_Date,

[code].....

I want o add additional column to this

I added this sql

SELECT CASE
WHEN currentstep.step_id IN (100)
THEN currentstep.start_date
ELSE (SELECT start_date
FROM audt.os_historystep
WHERE ID =

[code].....

here is the completed query

select EAG.AUDIT_NUMBER Audit_Nbr,
(
SELECT CASE
WHEN currentstep.step_id IN (100)
THEN currentstep.start_date
ELSE (SELECT start_date
FROM audt.os_historystep

[code].....

when I try select from this query I get ORA-00904: "DATE1": invalid identifier.

how to add the new column in group by clause.