The getmembers command returns a CSV list of agent names by UUID, representing all the members in the group.

llInstantMessage(CORRADE,
    wasKeyValueEncode(
        [
            "command", "getmembers",
            "group", wasURLEscape(GROUP),
            "password", wasURLEscape(PASSWORD),
            "callback", wasURLEscape(URL)
        ]
    )
);

• If the intent is to retrieve a list of group members that are reported to be online, then the getmembersonline command may be better suited.
• The output may be very large and the command may take some time to complete (or even timeout). In which case, Corrade's internal HTTP server can be used to obtain the complete list.