if you have a group based on the month, then you can use that to sum the numbers. You don't need to display the grouping, but it would need to exist.
Or you can write a formula like:
if month({table.datefield}) = month(now) and year({table.datefield}) = year(now) then
do something.
not sure how you want to see/display the value. if it is to be displayed in a group footer, you could add to a running variable using either running totals or shared variables (i like shared variables, DBlank is running totals)
HTH