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')})
Prepare form
dataset/ETHPy150Open treeio/treeio/treeio/core/forms.py/SubscribeForm.__init__