Running Total / cumulative by Group

Can't seem to get the handle on this.  Starting data:

Group     Service ID     Amount

   A               1                $2

   A               4                $1

   A               2                $3

   B               2                $1

   B               6                $4

Result Sought:

Group    Service ID    Amount   GroupSum
   A             1                 $2              $2
   A             4                 $1              $3
   A             2                 $3              $6

   B             2                 $1              $1
   B             6                 $4              $5

either Measure or Calculated column.... at Dax Patterns every example has a date field involved.....kind of going in circles on this one at the moment.......wondering if the lack of a sequential field is my issue or just general brain fog..........