To sign up for a mutuelle, you must:
Live in France (expats included)
Already be insured under the French public healthcare system with a permanent French social security number (“numéro de sécurité sociale définitif”)
You do not need to be a French citizen, just a resident covered by Assurance Maladie with a permanent SSN.
Do I need to be registered with the French public system first?
Yes. The mutuelle only works if you are already registered with Assurance Maladie (the French public health insurance) and have your permanent social security number. If you do not have your permanent number, you are not eligible.
Can I join if I am waiting for my social security number?
No. You must wait until your registration is complete and you have received your permanent social security number.
Do I need to have my Carte Vitale, or is the permanent SSN enough?
You do not need to have your Carte Vitale in hand to register for a mutuelle. As soon as you have your permanent social security number, you are eligible. The card itself is not required for sign-up, just the permanent number.
Declaring a “médecin traitant”
Once you’re insured, declare your primary doctor (“médecin traitant”) to get the highest possible reimbursement rates from the public system and your mutuelle.
Summary
You must live in France and already be covered under the French healthcare system with a permanent social security number.
You can’t join a mutuelle while waiting for your number.
The Carte Vitale is not required for registration, just your permanent SSN.
Declaring a médecin traitant is important for getting full benefits.