Complex sentence: Accel is also a backer of Groupon, which in late 2010 turned down an offer from Google to sell itself for some $6 billion and in early 2011 was said to be talking about an initial public offering which would value the company at $15 billion.
Split into simpler sentences: Accel is also a backer of Groupon, which in late 2010 turned down an purchase offer from Google for $6 billion. In early 2011 Groupon was talking about an initial public offering which would value the company at $15 billion.