If you want the phone you have in the US to work in Europe, you're going to want to pick a carrier that uses GSM technology. GSM is what most of the world uses for their cell phones networks.
In the US, T-Mobile and Cingular are GSM carriers. Virtually any of their phones will work in most spots in Europe. Both carriers have online maps that shows where in Europe they have coverage. Sprint and Verizon use CDMA technology. Only special, expensive versions of their phones (which actually have CDMA + GSM technology built into them) will work in Europe.
Both T-Mobile and Cingular will let you make/receive calls when you're in Europe, but their rates can be pretty expensive. Something also to be careful of if you take a T-Mobile and Cingular phone overseas is that if you get a phone call but don't answer it, you can still be billed a few minutes of airtime at whatever the international rate is where you're at, which can be very expensive ($2.99/minute or higher sometimes). Both carriers should have instructions on their website for how to avoid this.
Another option is when you get to Europe, you can buy a cheap prepaid phone. Or if you have a T-Mobile or Cingular phone, you can ask them to unlock it, and when you get overseas, you can pop a prepaid SIM into it. That's requires a little more understanding but is an easy solution too.
As for your family plan question, that varies by carrier. I know with T-Mobile, it's no problem to have a family plan but just pay extra for data (for the email) for just one of the phones.
Hope that helps some.
