There have been posts for this elsewhere on the site. You can search around for the details.

Basically, you have two options. First, if you don't need to show the Details section, then you can use a formula to concatenate the data together in a single string and then display the global variable in the group footer.

The second option, if you need to show the Details section, is to use a Shared variable with a subreport and in the subreport build the string with concatenation as the subreport processes the records. Then when the subreport is finished you can displayed the shared variable on the main page.