This is not a q to find out how to get Spiritomb in Platinum, I want to know if you have to have internet to talk to people in the Underground, as I'm pretty sure I remember it being said they're real people playing. Thanks!
Yes you can get Spiritomb without internet. You don't need to have internet to access a wireless connection to talk to actual people to get Spiritomb. Each time a player talks to another player, the counter on their card is incremented. This counter reaching 32 is one of the prerequisites for encountering Spiritomb.
I hope I've helped!