BELFAST, Maine — Five hundred seventy four people can really fill up a room — even the spacious atrium at athenahealth, Inc.

The company’s employees gathered Monday to take a photo to mark the health care information technology company’s 5th anniversary in Belfast. David Tassoni, vice president of operations, took the opportunity to give a little speech to the assembled workers.

“This is a really special day,” Tassoni told those gathered. “Five years ago, I watched 15 people walk in the door … The reason we’re here, the reason we thrive, is because of all of you.”

Company and local officials agreed that the anniversary this week, and what athenahealth has done in Belfast during its five years there, is something important for the whole area. The Watertown, Mass., based company has been hiring about 100 employees a year and is still actively recruiting more people, with a goal in mind of about 800 in the Belfast office.

“I think we’re well on our way,” Tassoni said.

Thomas Kittredge, the economic development director for the city of Belfast, had positive words for the growing company.

“It’s phenomenal to see the level of growth they’ve had in the last five years,” he said. “A business in this type of sector brings needed diversity to the Belfast and regional economy. We hope we can do whatever we can to make sure that new growth is not constrained. To see them go from zero to 574 in five years – that’s nothing short of dramatic.”

Staffers at the company’s Belfast operations center, located in former MBNA office space, work to sort and manage paperwork and billing for a growing stable of doctors and medical groups that make up the company’s clientele. Athenahealth founders Jonathan Bush and Todd Park started the company in 1997 to make the San Diego women’s health clinic they had just purchased profitable, but when they realized how hard it was to manage insurance company paperwork, they expanded and changed their mission. Now, the company, which employs more than 3,000 people in locations including Belfast, Georgia, India and Watertown, focuses on managing medical practices by processing insurance claims and patient payments.

“The work we have here for these folks is pretty complex work,” Tassoni said. “When we get a denial [from health insurance companies] in the door, we reverse-engineer that denial. Their job is to prevent that denial happening again in the future.”

Shares in athenahealth (ATHN) have been publicly traded on the NASDAQ since 2007. After the market’s close on Monday, stock in the company was listed at $95.50 per share. According to Dow Jones & Co.’s Marketwatch website, that came close to the 52-week high of $99.79. The low during that same period was $56.33.

Zach Means, a financial advisor at Means Investment Company in Bangor, wrote Monday that athenahealth is considered a “relatively aggressive growth” company, and that its recent acquisition of an important iPhone health care app earlier this year is “encouraging.” The $293 million purchase of Epocrates should allow athenahealth to garner more recognition by doctors.

“We really like the health care market in general,” Means wrote. “Athenahealth has put themselves in a better position to garner more of the health information services market, which should bode well long term.”

However, he added that his firm is concerned about the company’s valuation, and that its price to earnings ratio is about 70, which is much higher than most of its competitors. A ratio that high doesn’t always scare us, but it does make the stock expensive from a valuation standpoint, he said.

“We would not be buyers of the stock at this level,” Means wrote.

Neither Means Investment Company nor its employees own shares of athenahealth.

Wendy Coombs of Northport works in account management at the company. She was the fourth employee to be hired in Belfast and said Monday that the last few years have been exciting.

“We have a casual atmosphere with really great people working toward a great goal,” she said. “We’re here to make health care better for doctors, but at the end of the day, we want to make health care better for this nation. It’s a really honorable goal.”