As mentioned in this article, new members can join a group with installments even after one or more installments have been paid. When this happens, the participants that were already charged before the new members joined will be charged less for future installments. This ensures that everyone pays their fair share of the purchase in the end.
As an example: if there are three people in your group when you’re charged for the first installment, then two more people join before the next installment is charged, you’ll pay less for future installments due to the larger group size. Additionally, your ultimate share of the purchase will be based on a group size of five members, not three.
If the amount you overpaid due to new group members is large enough that reducing your future payments to zero still doesn’t get you to the correct contribution amount, Pay By Group will automatically refund you the difference between what you initially paid and your correct new share. Such refunds will be triggered at the same time as payment of the final installment.