I think you have three options here, given your desired workflow:
A. Before importing, delete the images you don't want to keep from the card, either in-camera or via a photo viewer on your computer. This makes me a little bit squeamish, not because this is likely to cause bugs or make the card go bad but because there's a lot of room for human error and you might delete something you didn't want. But, having done this, you then only import the photos you want.
B. Don't delete the unwanted images. Use the selection features in Lightroom to highlight the ones you want to keep and hide the unwanted ones. This uses more disk space, obviously, but, eh, disks are fairly cheap these days. You can even move the low-rated photos to a cheap external drive.
But I think overall, I think the best option is to find a different way to present your photos to your friends in the field. You could start accumulating them on a separate card, and swap that in if asked. Or, you could put them on your phone, which probably has a better, bigger screen and nicer photo viewing options anyway. The particularly nice thing is that then your friends are seeing your finished work, rather than just going through a huge number of unsorted snaps. That's probably nicer for everyone!