三人行，必有我师(焉)

I'm not sure what's happening at the end of this sentence grammatically.

There's a really common rhetorical-question construction in Cantonese that's similar (negation + X + question-y final particle), so that was the first thing I thought of when I read it, and based on what presumenothing said, it sounds like the meaning is the same -- I often see the Canto version translated as a negative rhetorical question like "is it not the case that X?" or "then wouldn't it be X?" or "then won't X happen?" (or just non-rhetorically as "then [obviously] X"). So maybe in this case something like, "just join the SID and you'll know, won't you?"