subreport, and in the subreport you will need to use a formula to sum the field A values. I would probably make a group in the subreport so that you can place the formula there.
Crystal has to read the records first, so the only way to get something like this in the report header is to have a subreport to force Crystal to read the data. This will imply that your data will be read twice. Once for the subreport and once for the rest of the report.
If you were getting your data from a stored proc, you could modify the stored proc to only report the value once per group then you could just do a sum of the field, which Crystal will perform in the report header.
HTH
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