There is really no good way to do this in Squarespace. Any monthly subscription is just a recurring monthly payment, so there is no notion of an "active subscriber" that you can view anywhere.
The best solution I've found is to go to the orders page and export everything to CSV for that particular product (you can filter by product during export), then import it into Excel and filter it to those who have paid within the last 30 days.
I would love it if the orders page itself added a column for the product and allowed us to filter the page directly by both date range and by product so I could see this information without exporting and doing this in Excel.
@paul2009 - are there any plans to add additional columns/filtering to the orders page to support this use case?