You can't do it in the mail app. You need a third party app like Group Email. You can check it out in the app store.
However, I did see one user suggesting to set up a list of email addresses in the notes app separated by commas I would imagine, Then copy that list and paste it into the To: field in the email and try that. The poster claimed that you will get a message to the effect that it is an invalid email address, but he claimed that it will go through anyway. You may need to put a space in between each email address as well if you try this.
EDIT - I tried this myself and I was able to do it without putting a space between the email addresses and just using a comma only. I tried it several times and it worked twice. Not the ideal way to do this but it is a workaround. I would use a third party app.
Message was edited by: Demo