'Prepare form'

def __init__(self, instance, *args, **kwargs):

    subscriptions = instance.subscribers.all()
    super(SubscribeForm, self).__init__(*args, **kwargs)
    self.subscriptions = subscriptions
    self.instance = instance
    self.fields['subscriber'].label = ''
    self.fields['subscriber'].queryset = User.objects.exclude(pk__in=subscriptions)
    self.fields['subscriber'].widget.attrs.update({'class': 'autocomplete', 'callback': reverse('identities_ajax_user_lookup')})
