Generally when implementing some "authentication logic" inside a django template it's enough to check if a user has some permissions, and in such cases you can use the variable perms, which is available if (from django site):

Technically, these variables are only made available in the template context if you use RequestContext and your "django.contrib.auth.context_processors.auth", which is default. For more, see the RequestContext docs.

But could happen that you need to check if a user belongs to a certain group.

In such case you may find some good custom templatetags like this, they work well and cover all your needs.

Another approach consists in writng a custom template filter and use it with the builtin templatetag if.
I've choosen this way.

So here comes a simple template filter which checks if the user belongs to a given group. It can be easily extended to take into account more groups.

from django import template
from django.contrib.auth.models import Group

register = template.Library()

@register.filter(name='has_group')
def has_group(user, group_name):
    group = Group.objects.get(name=group_name)
    return True if group in user.groups.all() else False

And in your template you can use it this way:

{% if request.user|has_group:"mygroup" %}
    <p>User belongs to my group
{% else %}
    <p>User doesn't belong to mygroup</p>
{% endif %}

That's quite easy and does what it has to.