serializador_notification.py

class MessageReadSerializer(ModelSerializer):
    users_receivers = serializers.SerializerMethodField()

    class Meta:
        model = ReceiverMessage
        fields = ['id', 'users_receivers', 'was_read']

    def get_users_receivers(self, instance):
        children = instance.user
        group_id = instance.group_id
        groups = []

        if children:
            return UsersReadSerializer(instance.user).data

        elif group_id:
            for children_group in UserGroup.objects.filter(group=group_id, user__role__name='Student'):
                groups.append(UsersReadSerializer(children_group.user).data)
                
            return groups

Si se manda return, es como un break y solo bota un resultado :V

Así fatima.

class MessageReadSerializer(ModelSerializer):
    send_to = serializers.SerializerMethodField()
    users_receivers = serializers.SerializerMethodField()

    class Meta:
        model = ReceiverMessage
        fields = ['id', 'users_receivers', 'send_to', 'was_read']

    def get_send_to(self, instance):
        if instance.user:
            return 'user'

        return 'group'

    def get_users_receivers(self, instance):
        if instance.user:
            users = User.objects.filter(pk=instance.user.id)
            return UsersReadSerializer(users, many=True).data
        elif instance.group:
            users_id = []
            for user_group in UserGroup.objects.filter(
                group=instance.group.id, user__role__name='Student'
            ):
                users_id.append(user_group.user.id)

            users = User.objects.filter(id__in=users_id)

            return UsersReadSerializer(users, many=True).data