@Chuan16 I have a friend who traveled in Thailand- she stayed at a guest house in Bangkok. She said one of the great things about the place, was that the guy who ran the guest house, who was a really ccol guy, let guests leave their luggage there while they travelled around other parts of Thailand.
So it can be an okay thing, but it's different in your case, because you don't have any experience with these people. Maybe after they stay with you, you can decide it's okay for them to leave their luggage, but meantime I'd do as others suggested- research luggage facilities or secure lockers in your area where they can leave their stuff. You can always decide after their first stay to let them leave it at your place, but I wouldn't make any promises at this point, or even mention that you might be okay with it.