Easily one of the silliest bugs in recent Destiny 2 history. In order to do this we had two 6 man fireteams in orbit. One fireteam loads the raid and when the launch timer hit 2 seconds the other fireteam leader joins resulting in a 12 man instance. This worked most consistently for us when the two fireteam leaders were geographically close to one another. Best of luck if you try your own 12 man PVE activity as this will likely be patched out very soon!
Timestamps
0:00 - Consecrated Mind (Garden of Salvation)
0:34 - Sanctified Mind (Garden of Salvation)
1:08 - Riven (Last Wish)
1:29 - Atraks-1 (Deep Stone Crypt)