In all my time trying for this thing, I never found a way to interact with other players in the Underground, with friends or otherwise. Mining with other players doesn't count for Spiritomb, entering their secret base doesn't work, even using the "Call" function while near another player yielded no results. My guess is it's something that was planned to be in the game, but didn't make the cut for some reason; I mean, this game's online is pretty barebones as-is, with link codes being barely functional and various online modes not even being done by release, so I wouldn't be surprised if the ability to interact with other players in the Underground was something that was planned at some point, but they just didn't have time for. Curiously, the website does specifically word the requirement for catching this thing as "Once you’ve finally spoken to 32 unique players or NPCs," so maybe it's a local-only feature currently? Because you definitely can't "speak" to other players in the Underground online as it stands.So the website has this whole thing about spiritomb, but
The wording of its entire section implies you can, infact, speak with other players to increment Spiritomb's counter which....isn't this wrong? Didn't someone determine you can't actually speak with other players during an online session?
Maybe it has to be in one of those friend rooms?
That said, if there IS an easier way to get this thing at a reasonable point in the story, I will gladly take it on a future playthrough.