Does this answer your question:
"The -j -U option enables the DBSA to grant individual users access to
the database server in administration mode. Once connected, these
individual users can execute any SQL or DDL command. When the server
is changed to administration mode, all sessions for users other than
user informix, the DBSA group users, and those identified in the
onmode -j -U command lose their database server connection. The
following example enables three individual users to connect to the
database server and have database server access until the database
server mode changes to offline, quiescent or online mode: onmode -j -U
karin,sarah,andrew"
InformixIBM Informix Dynamic Server Administrator’s Reference (IDS
11.50), p. 11-14
Group 'informix' should have no members aside from user 'informix'.
Or does this nostrum of Informix lore no longer hold?