sw
Ok...maybe I'm looking in the wrong place, but all I found was this (on the TI website):
Airport Transportation
What type of transportation is offered from the airport to Treasure Island?
Transportation is offered by Grayline (702) 739-5700 and Bell Transportation (702) 739-7990 for a nominal fee. Both shuttle companies pick-up outside of Baggage Claim at the McCarran International Airport.
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Doesn't even mention Greyhound.
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Also, I found this regarding the question of whether you need id:
"...identification is required upon check-in for all guests."