News of the pending round was first reported by Erin Griffith over at VC Experts, based on an amended and restated certificate of incorporation (view here). It does not appear that the money is yet in hand, and no new investor details are included.

Expect a Form D to be filed shortly with the SEC (which, again, does not necessarily signal a completed transaction).

From Griffith’s story:

There are a few key differences in terms and pricing between the Series G and its Series F, raised in April. The $135 million Series F, led by Digital Sky Technologies, was priced at $32.12 per share and was junior in liquidation preferences to all preferred shares. This round is priced $.53 less, at $31.59 per share and is senior to all. The latest filing also increased the authorized shares of voting common to 250 million shares, and if all of them are issued, Groupon’s valuation could be as high as $7.8 billion.

To date, Groupon has raised around $170 million in VC funding, from DST, Accel Partners, Battery Ventures and New Enterprise Associates.

Fortune has sent a request for comment to Groupon CEO Andrew Mason, and will update if we hear back.

In the meantime, some idle speculation as to what new investors could be involved (some original thinking, some help from my tweeps):

• Kleiner Perkins: How could Kleiner not be involved in this? Such deals are the very reason why it now has a digital growth fund, not to mention the related sFund. If landing Twitter was cache cake, then this would be the frosting.
• Andreesen Horowitz: They haven’t done too much yet out of their $650 million second fund, and perhaps this could be that vehicle’s Skype (an outsized deal that dominates the remaining portfolio)
• Goldman Sachs, Morgan Stanley, etc: There’s no better way to get the inside track to be Groupon’s IPO underwriter.
• Blackstone Group: Doesn’t do venture capital — outside of its cleantech fund — but is one of the few private equity firms that could write a $500 million check without blinking (it just closed a $12 billion fund, so it also doesn’t need to appease its investors for a while). Highly unlikely, I know. Maybe Warburg Pincus would make more sense, among the big buyout class (or perhaps tech-savvy firms like Silver Lake or Providence)
• Fidelity, T. Rowe Price: Not lead investors, but big money guys who could help flesh out the round
• Google: If you can’t buy ’em, join ’em

UPDATE: I’m hearing from a single — albeit typically reliable — source that Groupon was in talks to raise money from a Boston-based investment firm, prior to the Google offer. Also believe that same firm led this new round, at a significantly lower dollar amount than $950 million (but still more than 2x what Groupon has raised to date). Trying to confirm, at which point I’ll share more details...

Update II: Scratch the Boston-based firm thing. The firm was indeed in talks with Groupon prior to the Google offer, but is not involved in this round.

Update III: Reports now are that Groupon already has closed on at least half the money, with return backer DST being joined by Fidelity and T. Rowe Price. Fidelity is a Boston-based money manager, but not the one I was thinking of. On the other hand, glad to have included Fidelity and T. Rowe in the above list. No official word on how much Groupon has already banked, but the “significantly lower” amount I mentioned above was $450 million.