Related: Who Read
Showing who is actively reading is quite challenging because you need to use some sort of ping-based or presence-based solution.
Using Pusher and presence channels you should be able to make something quite quickly, but depending on your community size you would probably quickly go over Pusher free plan.
Another simple option might be to do something similar to Who Read and build the list from the Flarum read status table, but that wouldn't take into account guests or people reading older topics they have read before or reading the top of discussion without scrolling to the bottom.
