In France, all professional activities are categorized with an APE number. The activity of renting out falls under the category with APE number 68, where immatriculation and habilitation (ie registration and license) is required.
6810Z Activités des marchands de biens immobiliers
6820A Location de logements
6820B Location de terrains et d'autres biens immobiliers
6831Z Agences immobilières
6832A Administration d'immeubles et autres biens immobiliers
6832B Supports juridiques de gestion de patrimoine immobilier
It is not allowed to have an activity within a company which does not have an APE - number. Even if you do it as a private person, it is still a protected activity (like it is nog allowed as a private person to execute brain surgery ...).
It is simpel: If you organise the renting out for others, than you are either: touroperator, agence immobilier, agence commercial or real estate agent. It doesn't matter if you do this with or without earning a salary or a remuneration.
Documents : Categorie 68 / Insee