Groupon Inc's Lee Brown, who oversees the daily deal company's national sales, has decided to leave, following other senior executives who left the company this year, according to an internal memo obtained by Reuters on Thursday.

Brown, who was senior vice president of national sales and joined in 2010 from Yahoo Inc, will be replaced by Raj Ruparell, the memo from Groupon's operations chief Kal Raman said. A Groupon spokesman confirmed the changes and the accuracy of the memo.

Groupon has lost about three quarters of its market value since the company's IPO last year on concern that the growth of its main daily deal business is slowing.

There have also been reports of staff departures in recent weeks. The Wall Street Journal reported this week that Jayna Cooke, another top salesperson at Groupon, had left the company.

Brown was credited with the creation of Groupon's National Sales team, which arranges nationwide deals, rather than local offers.

"Under Lee's management, the team has secured a number of key customers, and I'd like to personally thank Lee for his contributions and wish him the best in his next opportunity," Raman wrote in the memo.

Ruparell, who is taking over the National Sales team, previously headed Groupon Goods, the company's fast-growing consumer products business, Raman added.

"Raj will report to Chris Muhr, SVP of Sales, and will be immediately focused on getting to know the team and delivering on our Q3 forecast," Raman added.