posts = Post.objects.filter(group=groupid)

all_posts = []

for p in posts:
    all_posts += [(x.id, x.marked_read_on, p) for x in p.unread_set.filter(user=request.user)]


#IN MY TEMPLATE
{%for unread_id, read_on, p in all_posts%}
{{p.title}} - {{read_on}}</br>
{%endfor%}