"The acquisition price is reportedly $120M, of which Microsoft would keep, as the largest shareholder, $80M. So their net acquisition cost is $40M.
Since Groove has raised over $155M, only Preferred shareholders will be get a pay out. A Boston Globe article mentions that "The complaint said Ozzie would get $382,966 under the deal, while Breyer would receive about $1.7 million and Accel about $3.4 million. Microsoft, Groove's largest investor, would recoup about $80 million, it said". The other major investor in the company was Intel Capital.
It sounds as though the most recent rounds were senior to previous ones (almost always the case), which explains why Microsoft is recouping most of its investment (Microsoft invested in the last two rounds of $38M in 2003, and $51M in 2001 according to VentureWire - sub req). Co-investors in these rounds would also get their money back. Investors in the $41M round that closed in 2000 would also have gotten a large portion of their investment back - provided that this round was senior to the others. Accel Partners, an early stage investor in the company, seems to only received a payout on its more recent follow-on investment."