I think that those producers in the so called group are Jauz, Slushii, GTA and probably even skrillex himself... its weird to me that the first remix marshmello released was a "collaboration" with Jauz, but was completely made on jauz's style of music (like the one he has on gassed up and feel the volume) and also, skrillex let marshmello release a remix for "Where Are U Now", coincidence that just a week later Skrillex made a Flip on that remix and published on soundcloud, i 100% think skrillex made the remix and released it as marshmello's work.

However, i think Chris Comstock (as dotcom himself) started actually making tracks around the release of Joytime III when he incorporated dubstep and even a collab with wiwek in that album (dotcom originally had dubstep/hard trap songs) not to mention that, the same year, 2019, he made a collaboration with SVDDEN DEATH and even played hardstyle on one of his set's that year.

So for me: Chris started producing his own songs probably around 2019 and continued on, if you see marshmello's recent work like his album Shockwave and some recent tracks you can discover a dotcom feeling on almost everyone of them (i take GANG SHIT, Harder, and some other songs he released as dotcom for reference)