I reckon the "group format" was introduced here in Oz sometime in 2003 around about the same time as Flight Number callsigns for domestics. I seem to recall that the initiative came from QANTAS at a forum of some sort and was quickly adopted as mentioned earlier under the guise of "World's best practice" IE because the Yanks did it it must be good.
The funny part is that the RAAF did not adopt it despite use of a common Manual of ATS. Things may have changed in the 2+ years since I retired!
J