I don't think its possible to reset GroupNumber instead there are two ways you can create the rank.

1. Using Formula
2. Using Running Total

Using Formula
    Create two formulas Group1 and Group2
    In Group 1 formula field
       place the following formula
            WhilePrintingRecords;
            Global numberVar group;
            group:=0;
       and place the same in the Section Group1. Also suppress the same

    In Group2 formula
       place the following formula

        WhilePrintingRecords;
        global numbervar group;
        group:=(group+1);
        group;
       Place the same in Group 2 section. Now you will get the result you want.


Using Running Total (Second Approach)
    Create one new Running total.

    Summary
        Field to Summarize    select the second group
        Type of Summary       select Distinct Count

    Evaluate
                                        For Each Record

    Reset
                                        Select First Group Field.