It's all in the metering. If you just meter the whole scene...the camera will include the background and give you an average...which will most likely blow out any white clothing.
On the other hand, if you just meter for the white clothing, then the camera will think it's a bright scene and give you settings that will underexpose.
If you don't have a light meter or a grey card, I would suggest that you take a reading off of your subject's faces. Put the camera into manual mode, get in close (maybe use spot metering as well) and adjust the settings to centre the 'needle' in the viewfinder. Depending on the tone of their skin, you may want to adjust it so that the needle is off of zero. For lighter skin, adjust it positive.
Then you can back up and shoot the whole scene...keeping the camera in manual. You may have to tweak it a bit but that's the idea.
If you are still blowing out the white clothing then adjust your exposure down until you are not blowing out the whites. Ideally, you probably want it so that the white clothes are just a notch below being blown out.