You need to create a formula which returns a group number based on the parameters. Then create a group using that formula field. Here is a close proximity to what you would need (you'll have to test and tweak it to make it perfect):

NumberVar Group;
NumberVar Counter;
Counter := FromRate;
Group := 0;
While Counter<ToRate Do
{
        //Incrementing to move to the next group

        Counter := Counter + Margin;

        Group := Group + 1;

    //If we found the group, then exit the loop by maximizing the counter
    If {table.field}<Counter Then
        Counter := 99999;
 }
//Return the group number
Group; 

Edited by BrianBischof - 25 Sep 2007 at 10:49am