You will need to use HDR or fill flash.

The scene as described has a contrast range that is more than your sensor can handle.

An ISO of 100 is too low for dusk (versus bright sunlight). If you’re going to use the A mode then move up to an ISO of 200 – your T1i might not be too grainy at 400. It’s a fairly decent quality camera.

Zoom in on the figures you want to take portraits of and activate your exposure lock, then zoom back out (within the timeframe your lock will ‘hold’), recompose and snap the shot.

How close are your figures? Are you using a fill flash with them? It will help make them stand out and brighten their faces.

Can’t tell if the “white” you’re talking about is a backlighting issue or reflections from sand. If the pic isn’t just right from bright sand reflections, you can fiddle with your exposure compensation to see how that helps.

proper exposure is universal (it does not depend on the camera)

Depends on if you want silhouette or if you want to see your subjects properly lighted. it also depends on where the sun is (is it directly behind your subjects, or is it off to the side out of the frame?).

I would test out different settings, as well as read a book on photography tips.

I personally would use fill-in flash. I took a picture of somebody at the beach during sunset with the sunset in the background. Since I did not want the sunset to be ‘washed out’ but I also wanted the person’s face to come out, I used flash (I don’t remember if I used Av mode or not). It’s one of my favorite pictures because it came out exactly how I was hoping it would.

Why would you worry about that-let the camera calculate it. The ISO is too low if you will try to do this at sunset by the way.

This is pretty obvious, and I hope you don’t feel that I am talking down to you, but if you’re going to have a white background, I would not go to the beach. Sand and salt air can ruin your camera.