Group, Individual... sounds like insurance
There are two ways to accomplish this. Both ways require an "onChange" on the first list box. When the individual selects from the drop down, onChange is fired.
The first method uses that JavaScript in onChange to submit the form back to itself, and then executes ASP to populate the second list box (basically refreshing the form with the second listbox populated with the values based upon the selection on the first listbox).
The second method, if you want to hide the page reload from the user, is to do this with IFrame in a hidden SPAN or DIV. When that onChange is fired, you change the URL of the IFrame to the address of an ASP page that will generate JavaScript to update the parent window's listbox (remember to attach some QueryString params to the new URL, or your ASP won't know what values get populated). This approach then makes it "seamless", but requires a bit more work, especially if this needs to be cross browser compliant.