Here's the official French website about how to welcome refugees from Ukraine (Google translation).
You have to contact your local Prefecture (French State Offices) :
"Welcoming refugees
If you would like to welcome Ukrainian refugees to your home, you can contact the association Aide médicale et caritative France Ukraine at this address.
You can contact your municipality or associations helping people in difficulty (Secours Populaire, Red Cross, Secours Catholique, etc.).
In addition, the Minister of the Interior asked the prefects to organize the coordination and the inventory in each department of the possibilities of aid for displaced Ukrainians. Mayors and individuals wishing to provide accommodation must contact the services of the prefecture. Some prefectures offer a form for declaring the availability of accommodation, you must indicate the municipality, the accommodation capacity in number of people and the number of rooms available."
Source :
https://www.service-public.fr/particuliers/actualites/A15542#:~:text=Si vous souhaitez accueillir des,Secours catholique...).