Groups allow you to easily manage your address book and send out bulk messages to a subset of people. You can create a group from the contacts in your account, there is an article on that here (link to add existing contacts to a group).
You can also create new groups to add existing or new contacts to any time!
Simply go to Contacts on the left hand side, and then to the Groups Tab. Hit ‘Add Group’ on the top left.
Give your group a name and hit save. You can edit, view, delete and send a message to any group from here. Just click on the icons on the right hand side!