It is a bit late to be starting to think about this now. What would happen if you think it isn't safe to give them a key? Are you going to stay home for their entire trip letting them in and out as they wish? How will a stranger here on the internet be able to tell you if it is safe?
If you give a key to each guest, some number of guests will lose them. Will you be comfortable with that? Some people put a realtor-type lockbox near their front door and give the guest a code to open it, use the key and put it back in right away. the code can be changed between guests.
Or you can get a fully electronic door lock with no key, just codes that you can customize for each guest.
This is something you should have thought about earlier!