I'm not sure nostr is actually really ready for a mass influx of people(?) unless you have quite a targeted ad campaign (I have suggested eg sponsoring a jay dyer as just and example which I think would work). It's quite a niche sorta place atm, sort of works as something that needs to be sought out. (?)

Someone did in fact buy a billboard space a while ago for a nostr and, I can't imagine anyone joined from it, but it looked nice lol.

Even let's say this guerilla ad ampaign from the bluesky user. It's unlikely that anyone seeing it irl will be convinced to joined. But it works as a very clickbait photo shared online. The actual irl ad is almost irrelevant.

I think that sort of thing could work. And is a lot cheaper lol