I don't think that you can, at least not simply, since you can't aggregate an aggregate...ie you can't get the SUM(DISTINCTCOUNT({table.field}, group))...at least I pretty sure that you can't. You could sum this value using shared variables, perhaps running totals, but it would have to be in the footer, not the header, as CR would need to read the data to get the answer, and it hasn't done that yet.
The one way around this, when joining to the tables directly would be to use a subreport...you could duplicate all grouping logic, place the summing in a formula, populate a shared variable and then access the shared variable in the main report.
If you are using a stored proc to drive your report, just create a new column and populate it...way simpler and is my preferred method of gathering data for reports, any reports.
HTH
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