SQL Server 2019

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create table CompanyGroup (SourceCompanyId int, TargetCompanyId   int);
insert into CompanyGroup values
 ( 4626 , 359468 )
,( 4626 , 7999   )
,( 56167, 11947  );
3 rows affected

create table Sales
( CompanyId int,  Product nvarchar(max), Sales int);
insert into Sales values
( 4626  , 'Monitors', 1000)

,( 4626  , 'Locks'   , 300)
,( 359468, 'Chairs'  , 500)
,( 359468, 'Lights'  , 300)
,( 7999  , 'Chairs'  , 500)
,( 4626  , 'Locks'   , 300)
,( 56167 , 'Locks'   , 1000)

,( 11947 , 'Tables'  , 300);



8 rows affected

SELECT
  COALESCE(c.SourceCompanyId, s.CompanyId)   AS GroupCompanyId,
  SUM(s.Sales)   AS Sales
FROM
  Sales          s
LEFT JOIN
  CompanyGroup   c
    ON c.TargetCompanyId = s.CompanyId
GROUP BY
  COALESCE(c.SourceCompanyId, s.CompanyId) 
  


GroupCompanyId
Sales


4626
2900

56167
1300