var data = [{ message: 'This is a test', from_user_id: 123, to_user_id: 567 }, { message: 'Another test.', from_user_id: 123, to_user_id: 567 }, { message: 'A third test.', from_user_id: '456', to_user_id: 567 }],
    groups = Object.create(null),
    result;

data.forEach(function (a) {
    groups[a.from_user_id] = groups[a.from_user_id] || [];
    groups[a.from_user_id].push(a);    
});

result = Object.keys(groups).map(function (k) {
    var temp = {};
    temp[k] = groups[k];
    return temp;
});

console.log(result);