You can't do this easily on the device, without an app to help you.
There are quite a number of group email apps available on the App Store. Most will only let you send email from inside their own app, but MailShot Pro creates special contacts that you can use from the Mail app and tens of thousands of compatible apps, building group email right into those apps.
MailShot can import entire groups from your address book with a few clicks, and turn them into a special contact.
You can find MailShot Pro at https://itunes.apple.com/us/app/mailshot-pro-group-email-done/id445996226?ls=1&m t=8. A free app is also available, which can only create a few smaller groups, but can be upgraded.
Disclosure: I am the developer of MailShot Pro and may benefit from its sale.