A recent interview of Tesla CEO Elon Musk about Bitcoin, vaccines, and space travel on social media app Clubhouse has made the service an even buzzier hangout for tech elites, celebrities, and investors.

Clubhouse, is an invitation-only audio app that lets users to speak to others or listen to their conversations. Since debuting last year, the app has quickly caught on.

Users downloaded Clubhouse more than 990,000 times in December, an increase of more than 1,280% over the previous month, according to mobile analytics firm Sensor Tower. And in January, the app’s downloads doubled again to more than 2.3 million.

Musk, who spent some of his time on Clubhouse Sunday night interviewing Robinhood CEO Vlad Tenev about the run up in shares of video game retailer GameStop, is just the latest big name to chime in on Clubhouse. The app has also attracted the likes of entrepreneur and investor Mark Cuban, comedian Tiffany Haddish, and Hollywood mogul Jeffery Katzenberg.

Here’s everything you need to know about Clubhouse.

What is Clubhouse?

The app combines a chatroom with the presentation of a live podcast. This lets users discuss matters of public interest like startup fundings and local politics. Though moderators usually plan who will be highlighted during a chat, the app’s tools allow for surprise visits, which has created impromptu conversations like the one between Musk and Tenev.

Another high-profile discussion on the app came when San Francisco District Attorney Chesa Boudin joined a conversation in mid-stream with tech industry insiders who were criticizing him for rising crime in the city.

The app is currently only available on Apple devices and not on Android.

How does Clubhouse work?

The app is invite-only, meaning someone who wants to join must be invited by someone who’s already on the service. Once in, users can hop into any room where people are chatting, create a new public room, or create a room for friends or specific users.

Creators of the room are automatically moderators, meaning they have the power to add, mute, or remove other speakers. Users who enter rooms as listeners and click a button to “raise their hands,” which notifies moderators that they want to speak. 

What are the rules?

Clubhouse users must go by their real names, although the app doesn’t appear to verify that information during registration. Harassment is banned as is hate speech, misinformation, and spam, but the app has struggled with policing bullying, homophobia, and anti-Semitism.

When it comes to the conversations, users aren’t supposed to publicly share their contents without first getting permission from the speakers. Users are also prohibited from uploading any content that violates intellectual property or proprietary rights.

But Clubhouse mostly relies on users to report violations, a strategy that has proven to be problematic on other social media services. The company says it records the audio of each room, and if a user reports a violation, the company can review that audio. Recordings are deleted if there are no complaints, the company says.

Who created Clubhouse?

Clubhouse was founded by Paul Davison and Rohan Seth, two Bay Area entrepreneurs and alumni of Stanford University. The company is based in Menlo Park, Calif., near San Francisco.

As of January, Clubhouse had raised a total of $110 million and is valued at $1 billion, according to PitchBook.

The company’s main investor is venture capital firm Andreessen Horowitz, which participated in $10 million Series A funding round in May and then led a $100 million round of funding last month. Other investors include venture capital firms Moxxie Ventures, Offline Ventures, SignalFire, and TechNexus Venture Collaborative, according to PitchBook. Angel investors include former Googler and Twitter executive Elad Gil, former AngelList CEO Naval Ravikant, and former CEO and cofounder of Epic Games-owned Houseparty Ben Rubin.