Thank you for your answer Xavier.
Unfortunately my problem was different. I had to use the command :
INSERT INTO res_groups_users_rel (gid,uid) VALUES (3,2);
Fortunately, it worked.
But now I can't access global settings from one of the companies ( multi company setup ) with this error in logs :
odoo.models: Impossible d'effectuer l'opération demandée, en raison des règles d'enregistrement : Type de document : crm.team, Opération : read, Enregistrements : 4, Utilisateur : 2
Which should look something like this in english :
odoo.models: Impossible to perform the requested operation, due to the registration rules: Document type: crm.team, Operation: read, Records: 4, User: 2
I can access the menu when i'm on the other company though which is very weird.