I would look very carefully into french law before doing such a thing.
There are so many things about french law that still feel rather alien to me, and I have no idea as to how this would be treated and what sort of contract you would need to have. A french one, that was water tight.
Who could make such a watertight contract up for you is beyond me as my dealings with notaires leaves me with the feeling that they do what they think and IF they get it wrong, that they know that there will be little retribution.
I say this as it has crossed my mind that these rent free people may feel like they are actually working for you and maybe even the state may think so too............. and perhaps you could end up having to pay them as house minders....... had you thought of that???
Also there are laws in France about kicking people out between Nov and March called the Treve Hivernale, nigh on impossible to do.
Frankly I would not do it.
I am not trying to be negative here, but practical. Sadly people end up in awful situations, too good natured and never imagining that X Y or Z could happen to them.