I am not an accountant, but I imagine it has to be booked as revenue in an accrual system on a per-period basis. To satisfy double-entry, it would originally be booked as cash and a liability.

Take magazine subscriptions. You pay $240 for an annual subscription to Frisbee Fancier's Magazine at the start of the year. They book this:

    Cash at Bank:    $240   -
    Magazines Owed:    -   $240

Then they send you the January edition ("Gold Plated Frisbee Showdown!") and do this:

    Revenue from Subscription: $20   -
    Magazines Owed:             -   $20

That is, they move $20 from liability to revenue. Cash at bank is unaffected by this transaction.

How Groupon chooses to recognise the timing of revenue will affect their apparent numbers. I would prefer a conservative magazine-style model as above, but it might be possible for them to book the revenue up front and then use that as their basis of their projections.

I don't know enough about Groupon or accounting to be certain. Seek professional advice before investing etc.