Unfortunately, for this type of thing, the only way it will work when you concatenate inside of Crystal is if it's in the footer.  Depending on your database, you could create a stored function that would return the concatenated value.   You would pass in some information from your data that would identify which invoices you want (key to the person/company being invoiced and possibly whatever other info you need to get the right data.)  Then, call the function in a SQL Expression using the fields from the report as your parameters.

Go back to your subreport idea - Does this really need to be in the detail or can it be in the header?  Link it based on the group info instead of the details, put the formula that concatenates the data in the subreport report footer and suppress all of the other sections. 

Hey Dell,