Group By -- all additional columns


I have a data set that looks something like this:

Rep Name --- Department --- Month 1 --- Month 2 --- Month 3 etc.

I want to do a 'group by' as some reps appear on multiple rows and I want to sum the Month total for those reps. That part is easy.

Is there a way to automate that all future months added also get that function (right now, I'd have to go in each month and add the new month to the list of columns being summed)? TIA