In order to keep track of which friend listened to each song, the playlist will include a profile picture on the songs as well. Friends Mix will update on Monday every week and include 25 tracks in each update, similar to the other Apple Music weekly playlists.
Friends Mix is making new use out of the social features that Apple debuted in iOS 11 last year, which lets users build their own Apple Music profiles and show friends the playlists and albums that they're listening to. This information is then displayed on a playlist or album's information page and within the "Friends Are Listening To" section in For You, and now it'll also be used to fill up the new Friends Mix.
Friends Mix is joining Favorites Mix (which updates on Tuesdays), New Music Mix (Fridays), and Chill Mix (Sundays) as the fourth Apple Music weekly playlist. Apple originally launched versions of New Music Mix and Favorites Mix in the iOS 10 public beta in September 2016, followed by Chill mix in June 2017, so it's been well over a year since we've seen a new weekly playlist debut.
Like the other playlists, you'll be able to find Friends Mix sitting atop the "For You" tab in Apple Music. It's unclear on which version of iOS Apple is rolling out the playlist, but the poster on Reddit does reference being on the iOS 12 public beta.
Otherwise, Apple Music isn't getting much of an overhaul in iOS 12, which is expected to launch in September. Subscribers will notice updated artist profiles with larger portraits on the iOS app, and Apple has already opened up a new "Coming Soon" section for users on iOS 11 as well.