var jsonPayload =
        {
            "language": "",
            "documents": [
                {
                    "id": "sample-contract",
                    "name": ""
                }
            ],
            "status": "DRAFT",
            "type": "PACKAGE",
            "roles": [
                {
                    "id": "Signer1",
                    "type": "SIGNER",
                    "signers": []/*{
                            "email": "[email protected]",
                            "firstName": "John",
                            "lastName": "Smith",
                            "id": "Signer1"
                        }*/

                    //"name": "Signer1"
                }
            ],
            "name": "Demande de signature(s)"
        }

// I add the signers like this

usersG.forEach(function(user) {
                    var new_signer = {
                        "email": user.email,
                        "firstName": user.firstname,
                        "lastName": user.lastname,
                        "id": "Signer1",
                        "language": user.language
                    };

jsonPayload.roles[0].signers.push(new_signer);