Not very familiar with PieFed, but from my experience on Mbin and Lemmy, communities only exist for another instance if either someone already follows it, someone click on a ! link to a community and follows it, the link of a post from a given community is searched in the instance's search field, someone comments or up/downvotes on a post, or, microblogging/Mbin-specific, someone boosts a post and is followed by someone from another instance. However, neither are fail-proof from what I tested, and from the little I used PieFed, it seems a bit more bureaucratic to find external stuff, which added to being the newer of the thread-type site engines, might make it a bit slower to find stuff initially.

Though, on this last part, similar to what I observe in Mbin, propagation is exponential, so at some point having to manually track stuff should become just a memory.