From tonight's build of 2.8 (2.8 b337+) you will be able to choose "Product" alongside "Sum", "Mean", "Range", etc. to apply to numeric fields when aggregating and as measures. You will need to click "Common Functions" in the function menu to reveal it. You will also be able to use SUBSET_PRODUCT in your formulae.

This is a somewhat specialist-use function and can result in very high, unusuable values. One typical use is investment analysis, where periodic returns (like 1.10 for 10% periodic return) need to be multiplied many times to obtain the return over much longer periods defined as subsets of rows using the SUBSET(X) clause of the the SUBSET_PRODUCT function.