Perhaps they could be introduced to the concept of a 'Ticket'.

Entry to many venues requires you to tender a ticket before entry, this may be bought beforehand or at the door.

In this case, and for people expecting to shelter with me, it means bringing something useful: food, drink, batteries, ham radio, whatever. It would be more acceptable if the 'Ticket' were enough to exceed their expected stay but anything showing an acknowledgement of their use of resources is acceptable.