It's actually not wi-fi. It's local connection so you have to be near people to communicate with people.
You can but not many
Sinnoh underground uses DS wirelless communications not Wi-Fi You need another DS and another Pokémon Diamond/pearl to use it completely
Find people underground that look like hunters and you can buy traps and goods with your spheres
Same thing you do in Diamond and Pearl, talk to 32 people underground.
You have to meet other people and take their flags in the Underground.
By fighting and traveling with the Pokemon, not letting the Pokemon get KO'd and feeding it poffins that it likes
You can get a Spiritomb after you insert the Odd Keystone into the broken tower (Hallowed Tower) and talked to 32 people underground.
Some people underground, they look like hikers, will sell you traps or goods for spheres, or they want to buy them.
you get a spirttomb by talking to people underground 32 times. P.S this will take a long time
You can find Spiritomb at Hallowed Tower Requires Odd Keystone and talking to 32 People total in Sinnoh Underground.
After you place odd keystone in broken stone tower, making it Hallowed Tower, greet 32 people in the underground. Afterward touch it and press A.
If oyu have a friend with diamond pearl or platinum.... you'll nedd to go in and out of bases showing each other your trainer cases