Online coupon company Groupon filed for an initial public offering of up to $750 million, the latest in a series of Internet companies to tap the U.S. capital markets. In April, a source told Reuters that Groupon could raise as much as $1 billion in the IPO, which could value the fast-growing daily deals site at $15 billion to $20 billion. The IPO filing did not specify the number of shares to be sold in the IPO, the price range, or the exchange, though it did say the shares would trade under the symbol “GRPN.”