Did more study, it turns out 'Group' is obsolete.

Per Greenplum 4.2 Administrator Guide:
   CREATE GROUP - p.433
      Description
         As of Greenplum Database release 2.2, CREATE GROUP has been replaced by CREATE ROLE, although it is still accepted for backwards compatibility.
      Compatibility
         There is no CREATE GROUP statement in the SQL standard.

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Greenplum 4.2 is based on PostgreSQL 8.2.
Per PostgreSQL 8.2 document:
   http://www.postgresql.org/docs/8.2/static/sql-creategroup.html
      Description
         CREATE GROUP is now an alias for CREATE ROLE.
      Compatibility
         There is no CREATE GROUP statement in the SQL standard.

CREATE USER is an alias for CREATE ROLE as well.

Per Greenplum 4.2 Administrator Guide:
   Description
      As of Greenplum Database release 2.2, CREATE USER has been replaced by CREATE ROLE, although it is still accepted for backwards compatibility.
   Compatibility
      There is no CREATE USER statement in the SQL standard.

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Per PostgreSQL 8.2 document:
http://www.postgresql.org/docs/8.2/static/sql-createuser.html
   Description
      CREATE USER is now an alias for CREATE ROLE. The only difference is that when the command is spelled CREATE USER, LOGIN is assumed by default, whereas NOLOGIN is assumed when the command is spelled CREATE ROLE.
   Compatibility
      The CREATE USER statement is a PostgreSQL extension. The SQL standard leaves the definition of users to the implementation.

Since both CREATE GROUP and CREATE USER are an alias for CREATE ROLE (see previous 2 comments).  Perhaps we could just follow PostgreSQL implementation for Greenplum and present ROLE as GROUP and USER (the way how current Greenplum implementation works); if you agree, then we can close this issue.

Discussed with Sachin, we will not take any action on this at the moment.