The MISSING option in the second PROC REPORT step includes the observations with missing values for the group variable. 
proc report data=grocmiss nowd headline missing;
   column sector manager N sales;
   define sector / group format=$sctrfmt.;
   define manager / group format=$mgrfmt.;
   define sales / format=dollar9.2;
   rbreak after / dol summarize;
run;