To create an email group in Gmail:
Go to contacts.google.com (Or, from any Google page, click on the 9 dots in the top right part of the page and choose "Contacts".)
In the left panel, find "Labels", and click on "Create Label" at the bottom of that section. Give your email group a name.
Once the label is created, use the search bar at the top to start typing the name of a person you want to add to the group.
Choose that person from the suggestions that pop-up by clicking on their name.
Click on the label icon (under their name and local time), click on the group (or groups!) you want to add them to, and click "Apply" at the bottom.
Repeat that process with all the group members. When you go to start an email in Gmail now, type the label name and it will pop up as an option and add the whole group.