YouTube’s experimental app Uptime, which lets you watch videos with friends while reacting and commenting, has now opened up to all users. The app was first launched in March of this year, from Google’s internal incubator, Area 120, as a means of testing a more interactive and social way to watch YouTube.

However, it initially required an invite to use it. That requirement was dropped a few days ago.

Instead of viewing YouTube videos on your own, then sharing those you like with friends via links in chats or to social networks, Uptime lets you watch videos with friends directly in its app. Friends can either co-watch with you in real-time, or they can join later to see others’ reactions to the videos played back as they watch, giving Uptime a lively and interactive feel even when you’re watching alone ego one mini.

The reactions – emoji you tap as you watch the video – are inspired by other live streaming video services, like Facebook Live or Twitter’s Periscope, for example. There’s a smiley face, surprised face, heart eyes, angry face, grimace, and crying face. You can also tap on the video to leave “sparkles,” but these will only be shown to those you’re watching with in real-time.

Since its launch earlier this year, Uptime has added new features, including a Facebook friend-finding function, support for watching music videos, and it has revamped its home screen to make it easier to find things to watch.

The idea to offer a co-watching experience is not unique to Uptime, however. It’s the hot idea du jour, with a number of apps moving into this space, including Tumblr’s Cabana, Let’s Watch It, Fam, and others. Even Skype has said this is a feature it plans to introduce in the future, starting with support for YouTube DataHOUSE.

The Uptime experiment is now one of several to emerge from Google’s Area 120, which lets entrepreneurial-minded Google employees try out new ideas. The program has already launched other projects, including a personal stylist app Tailor, a voice messenger Supersonic, and a learn-to-code app Grasshopper; another app, a salon booking tool called Appointments, has not yet launched publicly.

However, Area 120 apps aren’t branded under Google’s name in the App Store, nor does Google offer much help in terms of promotion. Instead, these apps have generally been spotted by reporters, outside of any sort of formal launch announcement.

Uptime hasn’t yet had a chance to gain many users because of this fact, and because it previously required an invite to test it out. It quietly dropped the invite code last week, which means anyone can download and use the app now. Despite these roadblocks, the app did manage to briefly snag a place on the App Store’s charts – at a peak of #403 in the Entertainment category, which has fallen since. That sounds unimpressive, but for an app that was basically in pre-launch mode and not open broadly, it’s worth noting that it ranked at all.

Uptime is a free download on the iOS App Store mathconcept.