"LinkedIn's own Web site contains hundreds of complaints regarding this practice," they said in the complaint, which also seeks unspecified damages.
Doug Madey, a spokesman for Mountain View, Calif.-based LinkedIn, said the company thinks the lawsuit is without merit and will fight it vigorously.
"LinkedIn is committed to putting our members first, which includes being transparent about how we protect and utilize our members' data," he said in an email Friday.
LinkedIn required the members to provide an external email address as their username on its site, using the information to access their external email accounts when they are left open, according to the complaint.
"LinkedIn pretends to be that user and downloads the email addresses contained anywhere in that account to LinkedIn's servers," they said. "LinkedIn is able to download these addresses without requesting the password for the external email accounts or obtaining users' consent."
LinkedIn software engineer Brian Guan described his role on the company's website as "devising hack schemes to make lots of $$$ with Java, Groovy and cunning at Team Money!" according to the complaint. Java is a programming language and computing platform released by Sun Microsystems in 1995. Groovy is a another language for the Java platform.
The plaintiffs, who are seeking a jury trial, provided a link to the engineer's post, www.linkedin.com/in/brianguan, which they said they last visited Sept. 13.
The customers blamed the hacking on LinkedIn's strategy, which they quoted from a regulatory filing, to "pursue initiatives that promote the viral growth of our member base," according to the complaint.
LinkedIn claims in press releases to have the largest online professional network with more than 238 million members, including executives from every Fortune 500 company.
Chief Executive Officer Jeff Weiner is quoted in the complaint saying on a second quarter earnings call, "This strong membership growth is due in large part to new growth optimization efforts."
One growth initiative was to send multiple e-mails endorsing its products, services, and brand to potential new users, following up with additional messages to people who didn't sign on, according to the complaint.
The existing users have no way to stop the process, they said.
"These 'endorsement emails' are sent to email addresses taken from LinkedIn users' external email accounts including the addresses of spouses, clients, opposing counsel, etc.," according to the complaint.
The actions were taken even though LinkedIn assures its users when they log in, "We will not email anyone without your permission," the plaintiffs said.
"LinkedIn's appropriation of e-mail addresses to send multiple reminder e-mails promoting its service is motivated by monetary gain," they said.