I have used formulas in these instances.
//@SalesGroup
if {table.SalesRep} in ["1","2","3"] then
"Sales Group 1"
else if {table.SalesRep} in ["4","5","9"] then
"Sales Group 2"
else if {table.SalesRep} in ["6","7","8"] then
"Sales Group 3"
else
"Sales Group X"
Click Insert->Group-> @SalesGroup
That's it