Initial Resource Consumer Group - How to change

Oracle DB: 12.1.0.2 - RAC

OS: OEL 7

I have noticed that after I created users, some users have "Initial Resource Consumer Group" as -  DEFAULT_CONSUMER_GROUP, CUST_GROUP, OPR_GROUP, SYSTEM_GROUP, etc. 

How would I change it for a particular users?

When we create users as below, how does the "Initial Resource Consumer Group", get assigned?

This is the syntax for two users

--DEFAULT_CONSUMER_GROUP

CREATE USER AJACKSON

  IDENTIFIED BY <password>

  DEFAULT TABLESPACE  USERS

  TEMPORARY TABLESPACE TEMP10

  PROFILE USERS_EMP2

  ACCOUNT UNLOCK;

  GRANT CREATE SESSION TO AJACKSON;

****************************************************

--OPR_GROUP

CREATE USER FPETERS

  IDENTIFIED BY <password>

  DEFAULT TABLESPACE  USERS