Create a running total to calculate the number of donors.

For example, running total for FY12:
Field to summarize: CnBio_ID
Type of summary: Distinct Count
Evaluate: use a formula: CnGf_1_Date in Date(2011, 07, 01) to Date(2012, 06, 30)
Reset: Never

Running total for FY13:
Field to summarize: CnBio_ID
Type of summary: Distinct Count
Evaluate: use a formula: CnGf_1_Date in Date(2012, 07, 01) to Date(2013, 06, 30)
Reset: Never

Place these running totals in the Report Footer section.