'Custom clean method.'

def clean(self):

    super(GroupInviteForm, self).clean()
    user = self.request.user
    is_manager = user.userprofile.is_manager
    is_curator = self.instance.curators.filter(id=user.userprofile.id).exists()
    if ((not is_curator) and (not is_manager)):
        msg = _(u'You    need    to    be    the    curator    of    this    group    before    inviting    someone    to    join.')
        self._errors['invites'] = self.error_class([msg])
        del self.cleaned_data['invites']
    return self.cleaned_data
