[Answered ]-How do I create and save dynamic fields in Django ModelAdmin?

The following code worked perfectly. The overridden __init__ and clean methods on the ModelForm adds the dynamic fields and defines how the values should be saved.

The overridden get_form and get_fieldsets together with the fieldsets attribute on the AdminModel make sure the dynamic form fields get displayed in the admin.

class PersonAdminForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(PersonAdminForm, self).__init__(*args, **kwargs)
        new_fields = {}
        initial = self.instance.groups.all()
        for group_type in GroupType.objects.all():
            field_name = '{0}_groups'.format(group_type.name.lower())
            qs = Group.objects.filter(group_type=group_type)
            field = forms.ModelMultipleChoiceField(
                queryset=qs,
                required=False,
                initial=initial,
            )
            new_fields[field_name] = field
        self.fields.update(new_fields)

    def clean(self):
        cleaned_data = super(PersonAdminForm, self).clean()
        groups = []
        for group_type in GroupType.objects.all():
            gt_name = '{0}_groups'.format(group_type.name.lower())
            groups.extend(cleaned_data.get(gt_name))
        self.instance.groups.clear()
        self.instance.groups.add(*groups)
        return cleaned_data

    class Meta:
        model = Person
        fields = '__all__'


@admin.register(Person)
class PersonAdmin(admin.ModelAdmin):
    form = PersonAdminForm

    # using the fieldsets attribute instead of fields in order to dynamically
    # add group type fields later.
    fieldsets = (
        (None, {
            'fields': (
                'name',
            ),
        }),
    )

    def get_form(self, request, obj=None, **kwargs):
        kwargs['fields'] = flatten_fieldsets(self.declared_fieldsets)
        return super(PersonAdmin, self).get_form(request, obj, **kwargs)

    def get_fieldsets(self, request, obj=None):
        fieldsets = super(PersonAdmin, self).get_fieldsets(request, obj)
        newfieldsets = list(fieldsets)
        fields = []
        for group_type in GroupType.objects.all():
            fields.append('{0}_groups'.format(group_type.name.lower()))
        newfieldsets.append(['Groups', {'fields': fields}])
        return newfieldsets