Two plaintiffs have joined a lawsuit against President Trump that accuses him of violating the Constitution by accepting payments from foreign governments through his business empire.

A nonprofit restaurant group and a woman who books events at hotels in Washington, D.C., claim they are hurt by Trump's unfair advantage in the marketplace, according to a court document filed Tuesday.

They argue that U.S. government workers and foreign dignitaries can divert their business to restaurants and hotels in which Trump has financial interests.

The lawsuit was filed in January by Citizens for Responsibility and Ethics in Washington, a government watchdog group. It claims Trump is prohibited from accepting anything of value from a foreign government by the Foreign Emoluments Clause.

An emolument is any kind of payment made to a federal official, according to the suit.

The Trump Organization deferred comment to the White House. White House officials did not immediately respond to a request for comment.

Related: Ethics group lawsuit says Trump is violating Constitution

Critics challenged the legitimacy of CREW's lawsuit because a person or organization must typically demonstrate they've been harmed -- a legal status known as "standing." CREW claimed that Trump is forcing them to divert time and money from its mission of "serving as a watchdog" to all parts of government.

The president is exempt from rules that require executive branch employees to sell business interests that could cause a conflict of interest. Trump has said he is turning over his businesses to his sons and others while he is in office.

Adding plaintiffs doesn't necessarily bolster CREW's standing, but it does draw more attention to the effort. Richard Painter, the organization's vice chairman, said the additional plaintiffs will make it harder for the government to sustain an argument that the case should be dismissed.

One of the new plaintiffs is Restaurant Opportunities Centers United, a group of 25,000 restaurant workers and restaurants.

The organization claims that Trump creates unfair competition when he receives payments from foreign or domestic entities, resulting in lost business, wages and tips for them.

Related: Restaurant alleges unfair competition in suit against Trump, DC hotel

The other plaintiff, Jill Phaneuf, made a similar argument. She books events for embassies and politicians at hotels managed by Kimpton in Washington, including the Carlyle Hotel and the Glover Park Hotel.

The suit says those properties compete with the Trump International Hotel, which Trump has an ownership stake in.

Phaneuf said her pay is directly tied to a percentage of the gross receipts of the events that she books for the hotels. Without relief from the court, she claimed, her salary takes a hit.

ROC and Phaneuf aren't the first in Washington to claim that Trump's luxury hotel gives him a leg up on the competition. Last month, a wine bar filed a lawsuit in D.C. Superior Court that made the argument.