A lot of people [on this forum] have had good success with - and reported good reviews of - the app MailShot (free for a limited group number/size, the a $3.99, USD, upgrade, if you wish). With it, you can create groups all day. And, you save those groups (with a name you choose) right in your Contacts app so you can use them in the Mail app. Easy peasy.
Disclaimer: I am one of those people who like it and who have had great success with it. YMMV.
Marilyn