The only way I know of to do this is to create a formula using a variable that gets updated for each record that is read.  This formula then has to be used in a group footer section in order to display the data.  The formula looks something like this:

stringvar mydata;
if OnFirstRecord or {group field} <> previous({groupfield}) then
  mydata := {table.mydatafield};
else
  mydata := mydata + ', ' + {table.mydatafield};
mydata

 

-Dell