public class Group {
    public String name;
    public String admin;
    public List<String> addedUsers;
    public List<String> invitedUsers;

    public Group(String name, String admin, ArrayList<String> addedUsers, ArrayList<String> invitedUsers) {
        this.name = name;
        this.admin = admin;
        this.addedUsers = addedUsers;
        this.invitedUsers = invitedUsers;
    }

    public Group() {
        // Default constructor required because we have a non-default constructor as well.
    }
}

DatabaseReference groupRef = ref.child("Groups");
ArrayList<String> addedUsers = new ArrayList<String>();
addedUsers.add("email1@gmail.com");
addedUsers.add("email2@gmail.com");
ArrayList<String> invitedUsers = new ArrayList<String>();
Group newGroup = new Group("GroupName",
        "email1@gmail.com", addedUsers, invitedUsers
);

groupRef.push().setValue(newGroup);