class Category(models.Model):
    name = models.CharField(max_length=150)
        
class Offer(models.Model):
    date_add = models.DateField(_(u'дата начала'),auto_now_add=True,)	
    title = models.CharField(_(u'название'),max_length=100,)    
    description = models.TextField(_(u'описание'),)
    categories = models.ManyToManyField(Category, verbose_name=_(u'категории'),)

class Join(models.Model):
    offer = models.ForeignKey(Offer,)    
    date_start = models.DateField(_(u'дата начала'),auto_now_add=True,)

object_list = Join.objects.select_related('offer')

 object_list = Join.objects.select_related('offer', 'offer__categories', )

{% for cat in join.offer.categories.all %}{{ cat }} {% endfor %}