Ten days ago, Microsoft launched built-in Beam streaming for the Xbox One to Insiders on the Preview Alpha ring. A week later, Alpha ring members got the Beam app as well, and despite a couple builds being released to Beta during that timeframe, Beam is still exclusive to Alpha.
That should change this week, according to Microsoft's Brad Rossetti and Beam co-founder Matt Salsamendi. According to Rossetti, Microsoft needs to push a build to Alpha first that will fix a multiplayer bug and luckily, that fix is already being internally tested.
Also, Salsamendi was the one that confirmed that Beta Insiders will be receiving both the built-in streaming service and the app.
The Xbox Insider Program is relatively new, and if you don't have a lot of experience with Microsoft's Insider Programs, it's important to note that nothing is set in stone here. The fix is slated to be released sometime this week, and if there are problems, it could get pushed to later this week. Beam doesn't come to Preview Beta until after that, which means that there's a real possibility that it could show up next week.
Of course, this information makes it clear that pushing Beam to the Beta ring is among the team's top priorities at the moment, so even if you do end up having to wait a bit longer, it won't be too long.