you actually need a table in order to create a group in your subreport and do the things you need to do. even if you're passing a parameter you still need records in order for this to work.
i'm assuming that you're trying to insert a crosstab in your subreport as well, correct? if so, the crosstab idea doesn't require a subreport...go back to your main report and try to insert a crosstab using the method outlined above and then you're done. the crosstab can use the records already on the main report and that's why it is a quick win when you need to list all of the values on the report in a different outline.