Running Total / cumulative by Group

Hi CahabaData you are true, lack of sequential field is the major issue.

You can achieve this using below steps.

1 -  Add a sequence number. You can use RANKX function for this.

Rank = RANKX(SampleData,SampleData[Service ID],,ASC,Dense)

2 - Add RunningTotal using below formula

RunningTotal =
CALCULATE (
    SUM ( SampleData[Amount] ),
    ALLEXCEPT ( SampleData, SampleData[Group] ),
    SampleData[Rank] <= EARLIER ( SampleData[Rank] )
)

This will give you desired results :)

CahabaData

You can first add an index column in Query Editor and then create a calculated column with EARLIER function to get the expected result.

GroupSum = 
SUMX (
    FILTER (
        Table1,
        EARLIER ( Table1[Group] ) = Table1[Group]
            && EARLIER ( Table1[Index] ) >= Table1[Index]
    ),
    Table1[Amount]
)

Best Regards,

Herbert