The easiest way is just to pass the band to the template. Templates are capable of navigating the relationships between models and there is both members and membership_set queryset managers on Group. So here is how I would do it:
view:
def group_details(request, group_id):
group = get_object_or_404(Group, pk=group_id)
return render_to_response('group_details.html',
{'group': group})
template:
<h2>{{ group.name }}</h2>
{% for membership in group.membership_set.all %}
<h3>{{ membership.person }}</h3>
{{ membership.date_joined }}
{% endfor %}