Right now we only support a single payment method for each invoice so a third party tool would be needed. The easiest way to do this may be to simply set up a paypal account to use to pay the invoice and simply have each person make their partial payment to that paypal account each month. You could even set up subscriptions in paypal so each person would be automatically charged by the main paypal account and then you would just need to use that account to pay your invoice.