I have found out the answer myself.
the command will be ==>
dspusrprf usrprf(grpxxx) type(*grpmbr)
Register Hereor login if you are already a member
There is no such command. There is no command for any objects on the system unless you've been given authority to use those objects (including possible 'special' authorities, e.g., *SECADM) or you have authority as an object creator.
In addition to being a "normal" user, if you have also been granted at least *USE authority to the group profile, you can use the DSPUSRPRF command against the group profile with TYPE(*GRPMBR) to list group members.