user.groups.add(1) # add by id

from django.contrib.auth.models import Group
my_group = Group.objects.get(name='my_group_name') 
my_group.user_set.add(your_user)

group = Group.objects.get(name='blogger')

users = group.user_set.all()

User.objects.filter(groups__name='blogger')

perm = Permission.objects.get(codename='blogger')

users = User.objects.filter(Q(groups__permissions=perm) | Q(user_permissions=perm) ).distinct()