How to change the group of the existing user

1. How to change the group of a user if the user is already exists. [oinstall to dba]

Sol: /usr/sbin/usermod -g dba oracle

2. If the oracle user exists, but its primary group is not "oinstall" or it is not a member of the "dba" group, then enter the following command:

Sol: /usr/sbin/usermod -g oinstall -G dba oracle