Okay, ZUN-san is obvious, and I know from descriptions elsewhere that Beat Mario-san is the guy in the Hawaiian shirt. So who's the big guy on the left?

Could he be the artist's self-insert?

One of Reitasai's organizers, forgot his name. This pic was drawn right in the middle of the livestream drinking party of Touhou 20th anniversary.

DAS58 said:

One of Reitasai's organizers, forgot his name.

Retaisai organizer Houjou also appeared in the program, but it's not him.
The guy on the left is Ban (伴), a manager of Niconico (Kadokawa-Dwango).

ddd666 said:

Retaisai organizer Houjou also appeared in the program, but it's not him.
The guy on the left is Ban (伴), a manager of Niconico (Kadokawa-Dwango).

Does his known identity merit creating a tag? Or is it not worth it, since this is likely the only time he'll show up?

Moonspeaker said:

Does his known identity merit creating a tag? Or is it not worth it, since this is likely the only time he'll show up?

Although I see him occasionally showing up in livestreams (e.g. nikenme-radio) and some events (mostly niconico-Touhou related events, since he's the guy responsible of them), I think the chances of him re-appearing in drawn-images are quite slim, if not zero.