im not 100% on this but i think the way it works is when you matchmake for a specific mission etc is searches for an available group, if all the groups are full it will then create a new one with you as the leader so others can join you
i dont think you can manually start one as the leader through matchmaking
That's my understanding too.
There is a way you could potentially force yourself. Set your matchmaking to private, start a group, and then change your privacy settings again.
There's no reason to want to be group leader though, as far as I know there's no advantages etc.
So how do I matchmake for a specific mission as I seem to be doing pretty much the same one over and over?