Good question, I am only recently coming to understand who they are purported be.
So far, what I can figure out is that, basically, the 'Sussex Squad' are so called 'superfans' of Prince Harry and Meghan Markle, who have in their efforts to support MM and PH, declare any ciriticism of them as being racist, they are hateful to anyone who doesn't like PH and MM, as I said they usually use race baiting in that effort, they have started despicable rumours about the Royal Family, in particular Prince William and Catherine, and it is becoming clearer, by the day, they are behind the vitriol against Princess Catherine during her cancer diagnosis.
Shockingly, It has since become apparent, that the Sussex Squad are allegedly very closely linked to Prince H and MM via this Christopher Bouzy guy, defacto head of Sussex Squad, who was a participant in their Netflix show, the Harvard lawyer who received a very nice donation from Archewell, all of who spew awful abuse and non facts about William and Catherine. There is also the recent visit to the home of a nephew of a Uvalde victim, who tags himself as Sussex Squad, as well as also writing awful mysogynistic and abusive tweets about random things (although he is only 21 yo in comparison to the much older Bouzy and Harvard head of Inclusion.) .
Prince Harry and MM have publicly thanked 'the Sussex Squad ' for contributions to their charities, (Sentabale) and have contributed funds to various efforts by the so called 'Sussex Squad'.
There is more, but it is more conjecture than facts, but it is looking like it will all eventually come out, maybe not today or tomorrow but there is certainly a view that it is all a very orchestrated effort that is not very far removed from H&M.