Scotrail said:First Class Caledonian Sleeper customers and Advance Loyalty Club members can enjoy complimentary light snacks and refreshments in the Virgin Trains First Class lounge. Dedicated Caledonian Sleeper Standard Single ticket holders can also be given access to the lounge. A voucher will be provided to those who qualify by your on train host.
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Standard Class customers can use the Passenger lounge, which is on the main concourse opposite the left luggage office. Both lounges are open daily.
I'm afraid it's not as simple as all sleeper passengers being able to use the 1st class lounge:
Dedicated Caledonian Sleeper Standard Single ticket holders can also be given access to the lounge. A voucher will be provided to those who qualify by your on train host.
But one can join the Scotrail advance club for free...
(It asks for a code off of a first class ticket valid in Scotland, but I just put in five random digits and it worked. Although it took about a month for them to send the card.)
So one can't actually join the Scotrail advance club for free then, at least non-fraudulently