This is not an easy thing to do with the stock e-mail app.
A lot of people on this forum (Disclaimer: myself included) have discovered and use the app Mail Shot. You can get a limited free version to try out (limited in that you can only make groups of five people) or buy the pro version for $3.99 (USD).
With it, you create a group, name it whatever you wish and it's stored in your Contacts app for an easy retrieval from the Mail app.
Good luck.
Marilyn