Theres an app for that, sort of.
There is a unique app called MailShot that lets you create single group contacts that can send email to many people at once.
When you have created a group in MailShot, you can then use a checkmark system to decide who is currently in the group, as you request, but to be honest its probably better to use a bunch of groups for different uses- all your mates interested in one subject, or the people you play your favourite sport with.
It may suit your needs, or it may not, but its free to try for a small number of groups and users, with an IAP available to address up to 50 groups of 50 people in each group. Check MailShot out on the app store now while its free.
Full disclosure- I am the developer of the app.