The person who organizes the study group often ends up understanding the material best.

Organising something forces you into the material sideways. You have to work out what order the topics go in, who is confused about what, and how to explain the hard bit three different ways, all of it more demanding than rereading the chapter. Teaching and arranging are a kind of study that does not look like study. The organiser gets the most out of the room while appearing to do the admin.

So volunteer for that job rather than waiting to be invited into a group. Book the room, send the message, decide what gets covered. You will end up knowing the material whether or not anybody turns up.