Select users to the group in group form on Django Admin.

group_users_form.py

# -*- coding: utf-8 -*-
from django.forms import ModelForm
from django import forms
from django.contrib.auth.models import Group, User

class GroupAdminForm(ModelForm):
    class Meta:
        model = Group
    
    group_users = forms.ModelMultipleChoiceField(label=u'Usuários deste Grupo', queryset=User.objects.all())

    def __init__(self, *args, **kwargs):
        super(GroupAdminForm, self).__init__(*args, **kwargs)
        users_id = list()
        # Avoid 'No exception supplied' 
        try: 
            users = self.instance.group.user_set.all()
            for u in users:
                users_id.append(u.id)
            if self.fields.has_key('group_users'):
                self.fields['group_users'].initial = users_id
        except Exception, e:
            pass

    def clean(self):
        group = self.cleaned_data['group']
        group.save()
        if group.user_set.all():
            group.user_set.clear()
        try:
            users = self.cleaned_data['group_users']
            for user in users:
                group.user_set.add(user)
        except:
            return self.cleaned_data
        return self.cleaned_data