I never played TSO, but I always thought the game would work well if it was like ... people hosted their own server (unless EA was willing to do dedicated servers for free...) and X amount of people could join said server ... nothing is different, except you all control different Sims. Maybe have voice enabled ... definitely a textbox for basic text chat with other players. That's about it. They could make it so that you have a separate Sim specifically for online play. Cheats would be disabled.
If that's too much trouble, then instead of having a place where a bunch of people can join random games, maybe you can generate a serial code or something that you can give to your friends so that people can join your game.
But in the end, I've always preferred The Sims as an offline game. At most, I would want to include up to 4 people, but that's about it. I liked games like There.com, Second Life, and Habbo instead, as they work much better as sandboxy social networking games.