Three UNIX groups should be added: dba, oper, and oinstall. The dba group is used for database install and has SYSDBA database admin privileges. The oper group is optional, this group maps to the SYSOPER privileges in the database. The oinstall must be created when you install the first time and is the owner of the Oracle Inventory which is a catalog of installed Oracle software on the system.
Only one user needs to be created: oracle. The oracle user owns all the Oracle software and it's primary group must be oinstall with dba and oper as secondary groups. After Oracle is installed you can change the primary group to dba, many people do, but it's not recommended.