This is a shot in the dark!!

I am not sure exactly what w want to do

It looks like a subquery would do the job...
I could be wrong -- I just give the gist...

SELECT [RO MTrak Report - Event Companies Distinct].REGION_CODE, Count([RO MTrak Report - Event Companies Distinct].COMPANY_ID) AS CountOfCOMPANY_ID
FROM [RO MTrak Report - Event Companies Distinct]
where [RO MTrak Report - Event Companies Distinct].REGION_CODE in (

select A.REGION_CODE, B.COMPANYID
from PROJ_INFO A, EVENT_PART B
where (B.ATTENDED = "Y" and A.EVCANCELLED = "N") AND
and A.ENDDATE between.....)

GROUP BY [RO MTrak Report - Event Companies Distinct].REGION_CODE;