From Mueller's filing on the GRU, Organization 1 is WikiLeaks:

The Grand Jury for the District of Columbia Charges:
Count 1
(Conspiracy to Commit an Offense Against the United States)
In or around 2016, the Russian Federation ("Russia") operated a military intelligence agency called the Main Intelligence Directorate of the General Staff ("GRU"). These units conducted large scale cyber operations to interfere with the 2016 U.S. presidential election. Defendants ... were GRU officers who knowingly and intentionally conspired ... to gain unauthorized access (to "hack") into the computers of U.S. persons and entities involved in the 2016 U.S. presidential election ... steal documents from those computers, and stage releases of the stolen documents to interfere with the 2016 U.S. presidential election. Starting in at least March 2016, the Conspirators used a variety of means to hack the email accounts of volunteers and employees of the U.S. presidential campaign of Hillary Clinton (the "Clinton Campaign"), including the email account of the Clinton Campaign's chairman. By in or around April 2016, the Conspirators also hacked into the computer networks of the Democratic Congressional Campaign Committee ("DCCC") and the Democratic National Committee ("DNC"). The Conspirators covertly monitored the computers of dozens of DCCC and DNC employees, implanted hundreds of files containing malicious computer code ("malware"), and stole emails and other documents from the DCCC and DNC. By in or around April 2016, the Conspirators began to plan the release of materials stolen from the Clinton Campaign, DCCC, and DNC. Beginning in or around June 2016, the Conspirators staged and released tens of thousands of the stolen emails and documents. They did so using fictitious online personas, including "DCLeaks" and "Guccifer 2.0." The Conspirators also used the Guccifer 2.0 persona to release additional stolen documents through a website maintained by an organization ("Organization 1"), that had previously posted documents stolen from U.S. persons, entities, and the U.S. government.

pg 17
Use of Organization 1:
In order to expand their interference in the 2016 U.S. presidential election, the Conspirators transferred many of the documents they stole from the DNC and the chairman of the Clinton Campaign to Organization 1. The Conspirators, posing as Guccifer 2.0, discussed the release of the stolen documents and the timing of those releases with Organization 1 to heighten their impact on the 2016 U.S. presidential election. On or about June 22, 2016, Organization 1 sent a private message to Guccifer 2.0 to "[s]end any new material [stolen from the DNC] here for us to review and it will have a much higher impact than what you are doing." On or about July 6, 2016, Organization 1 added, "if you have anything hillary related we want it in the next tweo [sic] days preferable [sic] because the DNC [Democratic National Convention] is approaching and she will solidify bernie supporters behind her after." The Conspirators responded, "ok...i see." Organization 1 explained, "we think trump has only a 25% chance of winning against hillary...so conflict between bernie and hillary is interesting." On or about July 14, 2016, the Conspirators, posing as Guccifer 2.0, sent Organization 1 an email with an attachment titled "wk dnc link1.txt.gpg." The Conspirators explained to Organization 1 that the encrypted file container instructions on how to access an online archive of stolen DNC documents. On or about July 22, 29016, Organization 1 released over 20,000 emails and other documents stolen from the DNC network by the Conspirators. This release occurred approximately three days before the start of the Democratic National Convention. On or about October 7, 2016, Organization 1 released the first set of emails from the chairman of the Clinton Campaign that had been stolen by LUKASHEV and his co-conspirators. Between on or about October 7, 2016 and November 7, 2016, Organization 1 released approximately thirty-three tranches of documents that had been stolen from the chairman of the Clinton Campaign. In total, over 50,000 stolen documents were released.

Conversations reported from WikiLeaks to DJT JR.
"Strongly suggest your dad tweets this link if he mentions us," WikiLeaks went on, pointing Trump Jr. to the link wlsearch.tk, which it said would help Trump's followers dig through the trove of stolen documents and find stories. "There's many great stories the press are missing and we're sure some of your follows [sic] will find it," WikiLeaks went on. "Btw we just released Podesta Emails Part 4."

WikiLeaks Twitter account sent a private direct message to Donald Trump Jr., the Republican nominee's oldest son and campaign surrogate. "A PAC run anti-TRump site putintrump.org is about to launch," WikiLeaks wrote. "The PAC is a recycled pro-Iraq war PAC. We have guessed the password. It is 'putintrump.'"

Wikileaks urged Donald Trump Jr. to tell his father to not concede if he lost on Election Day, according to a bombshell report from The Atlantic that alleges the president's eldest child and the transparency organization secretly corresponded in private direct messages on Twitter during the U.S. presidential campaign.