Property tax (if through rent), sales tax, gas tax, income tax (unless the employer is breaking the law), social security tax (unless the employer is breaking the law), medicare tax (unless the employer is breaking the law). If they are here they are contributing even if you chose to ignore the fact.
If they live in government subsidized housing --- which they very often do -- and with several families in a one family apartment, and they aren't paying health insurance premiums (how many bother to do that???), and they aren't paying their own school tuition, they are not contributing. If they collect food stamps and welfare checks, have unpaid hospital and doctor bills they certainly are not contributing.