Ok delete the stored proc you initially created and change the line starting select cmd@1 to the below
SELECT @CMD1 = 'GRANT EXEC ON ' + '[' + @OwnerName + ']' + '.' + '[' + @ObjectName + ']' + ' TO [' + @user +']'
recreate the stored proc with this amendment and then use below to execute
exec spGrantExectoAllStoredProcs 'semhc\domain users'
Again just to reiterate you will be giving access to all stored procs to all domain users which is a little risky!