If there are 4 families and each is expected to pay $600, then the cost is 4*$600= $2400.

If the family can only pay $500, then $2400/$500= 4.8 families.

However, you can't have a fraction of a family so 5 families are needed, if the family can only pay $500.

Therefore, 5-4 =1 more family needs to sign up.