Django many-to-many :how to get the id

I have models with manytomany field:

class Brand(models.Model):
    Company_Group = models.ManyToManyField(Company)

class Company(models.Model):
    Pref_Company_Name_Flg = models.CharField(u'Preferred Name Flag',max_length=255, default="")
    Pref_Company_Name = models.CharField(u'Preferred Name',max_length=255, default="")

I want to filter the id of brands in containing "company_instance"

brands = Brand.objects.all()
company_instance=company.objects.filter(id =company_id)
for brand in brands:
    for i in Brand.Company_Group.through.objects.filter(Company_Group = brand):
        print i.id

I have found the Similar problem as follows: click here

but Report errors :

Cannot resolve keyword 'Company_Group' into field. Choices are: brand, brand_id, company, company_id, id

then I try this way,also Report errors:

type object 'Brand' has no attribute 'Company_id'

Thanks andvance