sql query question

Hi everyone i'm new in the forum, so nice to meet everyone
i have several questions for you but first i need your help with one

i got this query
-- consulta Resumida CRL
select c2.URL as CRLDP_URL,c1.FechaActualizacion as CRL_Fecha_Actualizacion,
c1.SiguienteActualizacion as CRL_Siguiente_Actualizacion,
c1.CRLID,
(select count(*) from entradacrl where pofFirmaDigitalID=e.pofFirmaDigitalID) as Cantidad_Entradas_CRL,
(select CommonName from persona inner join Certificado on PersonaID=pofPersonaID
where FirmaDigitalID=c.toFirmaDigitalID) as Titular_Emisor

from firmadigital fd inner join certificado c on fd.FirmaDigitalID=c.ToFirmaDigitalID
inner join crl c1 on c1.FirmaDigitalID=fd.FirmaDigitalID
left outer join crldp c2 on c2.CRLDPID=c1.ToCRLDPID
inner join persona p on p.PersonaID=c.pofPersonaID
left outer join entradacrl e on e.pofFirmaDigitalID=c1.FirmaDigitalID

where c1.FechaActualizacion between '1/5/1950' and '4/5/2050'
and c.ToFirmaDigitalID=(select c.toFirmaDigitalID from persona inner join Certificado on PersonaID=pofPersonaID
where FirmaDigitalID=c.toFirmaDigitalID and commonname='prueba6')
group by c2.URL,c1.FechaActualizacion,c1.SiguienteActualiza cion, c1.CRLID, e.pofFirmaDigitalID,c.toFirmaDigitalID, Titular_Emisor

my problem is that the last part of the group by clausule (Titular_Emisor) gave me an error because sql doesn't recognize it, how can I include that Column in the group by?

thank very much