PLS TRY BELOW CODE

CREATE TABLE #Companies (CompanyID int)

INSERT INTO #Companies (CompanyID) VALUES (1)

INSERT INTO #Companies (CompanyID) VALUES (2)

INSERT INTO #Companies (CompanyID) VALUES (3)

CREATE TABLE #CompanyGroups (CompanyGroupID char(1), CompanyID int)

INSERT INTO #CompanyGroups (CompanyGroupID,CompanyID) VALUES ('A',1)

INSERT INTO #CompanyGroups (CompanyGroupID,CompanyID) VALUES ('A',2)

DECLARE @CompanyID int

DECLARE @CompanyGroupID char(1)

--SET @CompanyID = 1

--SET @CompanyGroupID = NULL

SET @CompanyID = NULL

SET @CompanyGroupID = 'A'

SELECT * FROM #Companies WHERE CompanyID = (CASE WHEN @CompanyID IS NULL THEN CompanyID ELSE @CompanyID END)

AND CompanyID IN(Select CompanyID FROM #CompanyGroups WHERE CompanyGroupID = CASE WHEN @CompanyGroupID IS NULL THEN CompanyGroupID ELSE @CompanyGroupID END)