There are probably some limitations, but as a practical matter, if the market will buy they can always issue more stock.

In the case of Groupon, i dont know the specific, but the sellers of the stock were probably the current owners, not the company itself. Which is often, but not always, the case with IPO’s. It’s very possible the company didn’t raise a dime. (As I said, I don’t know the specifics, it’s also possible that the entire proceeds went to the company.) Barring some sort of buyout, the next time the sellers want cash, they can sell shares on the open market. (Often there’s some sort of waiting period, 18 months, two years or something, where the original stockholders can’t sell shares.)

The alternative would be for the company to raise money by selling new shares. Number of shares goes up, market cap goes up by cash raised, value per share theoretically stays the same.