We are travelling from Venice to Bellagio. I know I can get to Como by train from Milan, but then what? There will be 8 of us, any suggestions.

It's been awhile, but I believe you will have to cab it (5 min) to the hydrofoil in Como. The ride to Bellagio from there is about 2 hours I think. Not sure about driving from Como to Bellagio, may take much longer as there is no straight highway route. You might want to spend a night in como after your 3 hour train ride from venice to Milan, and then your hour ride from milan to como and all the waiting in between. Otherwise it will be a truly exhausting day.

If the goal is Bellagio and not be in Como, you can train from Milan to Verenna and then boat (10 minutes) to Bellagio.

The train from Milan to Varenna is 1 hour.

We were in Bellagio a year and a half ago. The hydrofoil ride from Como to Bellagio is only about 45 minutes (at most) long. We arrived at the train station in the late evening, and the boats were no longer running. We took a taxi ride from the train station to Bellagio. It was very expensive and quite a "twisty" road. I actually got a little car sick! If you can, definitely take the hydrofoil.

To clear up the confusion about the time it takes to get from Como to Bellagio by hydrofoil, there are two different schedules: The "express" only makes 2 or 3 stops and takes 45 minutes, while the "local" make many stops and takes around 2 hours.
Myer is quite right about the train to Varenna. That's the much better and faster way.

Does the train from Milan to Varenna stop anywhere near Malpensa airport? I'd like to catch the train north of Milan, rather than backtracking from the airport to Milan Centrale train station?

The train from Milano Centrale to Varenna does not travel anywhere near Malpensa.

There is bus service between the city of Como and Malpensa; the same bus also stops at Menaggio which is just across the lake from Bellagio but it is faster to travel between Como and Bellagio by hydrofoil.

This site has links to Bellagio/Lake Como hydrofoil and other transportation information:

http://www.bellagiolakecomo.com/travelinfo.html