Groupon Inc opened the IPO window a crack earlier this month and now companies, including Yelp Inc, Angie's List and even social-gaming giant Zynga, are climbing through it, too.

Yelp Inc, which offers online reviews of local businesses and services, filed on Thursday afternoon for an initial public offering of up to $100 million of Class A common shares.

A portion of the shares would be issued by Yelp, while the rest would be sold by some stockholders.

That followed the stock market debut of Angie's List Inc, another online provider of reviews for local services. Its shares surged 25 percent to close just over $16 on Thursday.

Groupon pulled off one of the biggest Internet IPOs in years on November 4, raising more than $700 million, despite questions about its business model, accounting and management. The deal ended a drought of public offerings since the summer, when stock markets slumped.

When underwriters see a window like this, whether really open or not, they just stuff the IPO channel, said Scott Sweet of research firm IPO Boutique.

There are eight IPOs that have either already happened or are scheduled for this week, putting it on course to be the busiest week since 2010, Sweet noted.

Yelp, which features more than 22 million reviews of businesses ranging from dentists to restaurants to plumbers, claimed to have 61 million unique visitors on a monthly average basis in its latest quarter, according to a regulatory filing.

The company has not picked an exchange to list its shares on yet, but plans to trade under the symbol YELP, according to a preliminary prospectus filed with the U.S. Securities and Exchange Commission.

Earlier this month, the Wall Street Journal reported that Yelp was planning an IPO that could value the company at up to $2 billion.

Yelp was thought to need a new Chief Executive before it went public. In July, the company hired Rob Krolik, formerly CFO of Move.com, who has IPO experience from taking Shopping.com public and selling it to eBay Inc.

Goldman, Sachs & Co will be the lead bookrunning manager for the offering. Citigroup Global Markets Inc and Jefferies & Company Inc will be joint bookrunning managers.

Zynga is expected to go public after Thanksgiving. On Thursday, the company updated its IPO filing with the SEC, disclosing some executive and board changes. Zynga investor Brad Feld, managing director of venture capital firm Foundry Group, is leaving Zynga's board. Such moves are common before companies go public.

The amount of money a company says it plans to raise in its first IPO filings is used to calculate registration fees. The final size of the IPO can be different.

(Reporting by Brenton Cordeiro in Bangalore and Alistair Barr and Sarah McBride in San Francisco; Editing by Joyjeet Das)

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