My 24-year old son and his wife live with us; they both work. We don't charge rent because they are very frugal and are saving up for a house. They use public transportation, do not drink at all or go to restaurants often; DIL loves thrift stores, etc.
We have a phone plan that is just $100/month for 5 phones, so we just pay that (dd lives in an apartment with one of the phones.) DS and DIL help out so much, and we are so happy to have them here. All our yardwork, housework, and most of the cooking is done by the two of them. We share in the groceries; we shop frugally and are vegetarian, so food is cheap. We pay utilities, and they pay for the wifi (no cable,we cut the cord).
Dh is disabled, so the help is very welcome; we love them, and we are happy to have them stay as long as they want.