If you regularly send messages to the same set of people, you can create a group for them in the Contacts app, then enter the group name in the address field of your message. If Mail in El Capitan quits when you enter the group name, make sure that you've updated to OS X El Capitan v10.11.2.
You can also use one of these other ways to add the group:
- Click the Add button in the address field, then choose your group from the list of contacts.
- Type the full group name and press Return, instead of allowing Mail to complete the group name for you.