Assign User-objects to a Group while editing Group-object in Django admin

The save method above won't work if you add a new group and simultaneously add users to the group. The problem is that the new group won't get saved (the admin uses commit=False) and won't have a primary key. Since the purpose of save_m2m() is to allow the calling view to handle saving m2m objects, I made a save object that wraps the old save_m2m method in a new method.

def save(self, commit=True):
    group = super(GroupAdminForm, self).save(commit=commit)

    if commit:
        group.user_set = self.cleaned_data['users']
    else:
        old_save_m2m = self.save_m2m
        def new_save_m2m():
            old_save_m2m()
            group.user_set = self.cleaned_data['users']
        self.save_m2m = new_save_m2m
    return group

yourapp/admin.py

from django import forms
from django.contrib import admin
from django.utils.translation import ugettext_lazy as _
from django.contrib.admin.widgets import FilteredSelectMultiple

from django.contrib.auth.models import User, Group

class GroupAdminForm(forms.ModelForm):
    users = forms.ModelMultipleChoiceField(
        queryset=User.objects.all(), 
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name=_('Users'),
            is_stacked=False
        )
    )

    class Meta:
        model = Group

    def __init__(self, *args, **kwargs):
        super(GroupAdminForm, self).__init__(*args, **kwargs)

        if self.instance and self.instance.pk:
            self.fields['users'].initial = self.instance.users.all()

    def save(self, commit=True):
        group = super(GroupAdminForm, self).save(commit=False)

        if commit:
            group.save()

        if group.pk:
            group.users = self.cleaned_data['users']
            self.save_m2m()

        return group

class GroupAdmin(admin.ModelAdmin):
    form = GroupAdminForm

admin.site.unregister(Group)
admin.site.register(Group, GroupAdmin)