Or have a total population metric, some communities are more spread out. So the heard would be people within x radius or increase radius x until population is met.
At some point that's just going to become a subreddit with a very, very loose sense of geography built into it that's more of a hindrance than an asset. For this type of thing, you either go all in like yik yak did with the geo or you go all in with the idea of the community and non-geo (reddit).