I'm not entirely sure—since, as I said, I don't use Twitter. My main guess, however, is that it's broadcast.

Those other places you have to go looking for. If you're on Twitter, and already following some people within your groups, you're likely to see them retweet other people in the group, and over time build out your network that way. I would presume the algorithm will also notice your preference for such accounts, and recommend others, but without personal experience with the Twitter algorithm, that's also just a guess.