Attempt at replicating https://softwarecrafts.co.uk/100-words/day-119 with plain prefetches

client_list.html

<table>
{% for client in clients %}
    ...
    <tr>
    ...
        <td>
            <select>
                {% for group in client.groups.all %}{# prefetched #}
                    <option{% if group.pk in user_groups %} selected{% endif %}>{{group}}</option>
                {% endfor %}
            </select>
        </td>
    </tr>
    ...
{% endfor %}
</table>

models.py

# reversed-engineered from your example
from django.db import models

class Client(models.Model):
    pass


class Group(models.Model):
    client = models.ForeignKey("Client", on_delete=models.CASCADE, related_name="groups")


class User(models.Model):
    client = models.ForeignKey("Client", on_delete=models.CASCADE, related_name="users")
    groups = models.ManyToManyField("Group")

views.py

from django.shortcuts import render

from .models import Client


def client_list(request):
    return render(
        request,
        "client_list.html",
        {
            "clients": Client.objects.prefetch_related("groups"),
            "user_groups": set(request.user.groups.values_list("pk", flat=True)),
        }
    )