models.py

from django.db import models

class Item(models.Model):
  title = models.CharField(max_length=128)
  
class GroupItems(models.Model):
  group = models.ForeignKey(Group)
  item = models.ForeignKey(Item)

class Group(models.Model):
  items = models.ManyToManyField(Item, through='GroupItems')
  seleted_item = models.ForeignKey(GroupItems)