I posed this question becuase the answer was really hard to find. If silph co has no people in it at all it's because you have rotom in your party. Silph co's main purpose is to transform rotom, but when you enter the building with him in your party the place is vacant. All you have to do is go in without him in your party and there will at least be a man at the desk.
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You can't.
In HeartGold and SoulSilver, you can access the 1st floor of Silph Co. as soon as you enter Saffron City however you can't go into the elevator in order to get to the 2nd floor unless you have a Rotom as your lead Pokemon.
You need to get latias ( can only catch it on Pokemon heartgold) and latios (can only get it on Pokemon soulsilver) and speak to steven who is in the silph co. in saffron city.You must get the Enigma Stone from the WiFi event, which turns out to be the Soul Dew.
You can find Steven in both HeartGold and SoulSilver. He is in Saffron City. He comes towards you after you retrieve Copycat's doll from the Pokémon Fan Club building after you talk to Copycat. Once you leave the building, he comes and tells you about Latios (in SoulSilver) or Latias (in HeartGold). Afterwards he can be found in the Silph. Co. building in Saffron City.
Use the Rotom you caught in diamond, pearl and platinum.