I called Grey Line (702-739-5700), and they said they would shuttle for $4.75 one way. However, I didn't get a strong feeling that the person on the other end knew what she was talking about, or she was trying to get me off of the line. Anyone else wanting to use them should call and double-check. None of the other companies listed offer service to the Marriott. Evidently 20 minutes is too far away. I'll keep looking.
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>When I was in LasVegas last summer there were several shuttle companies operating out of the airport. Follow the amples signs for "shuttle". I recall it being about $12 to the strip, but this website (listing the companies) shows it as much cheaper.
http://www.a2zlasvegas.com/transportation/airshutt.html>
>The shuttles were stopping even at tiny fleabag motels off the strip, so I'd be shocked if they didn't go to the Marriot. You can ask the shuttle drivers when you get there or phone the companies on that weblink.