You do this via a checkbox in Teams Settings. Here’s how:
Select your profile pic at the top of the Teams app, then Settings > General. (General should be what’s showing when you open Settings, so you probably won’t have to select it.)
Select Turn on new meeting experience.
Restart Teams by doing the following:
a. Right-click or Cmd-click the Teams icon in the Windows task bar or Mac System Tray.
b. Select Quit.
c. Start Teams again like you normally would.
You might also see a notification that announces the new experience or reminds you to turn it on. Then it’s even easier—in that notification, choose Turn it on now if you’re ready at that point, or Maybe later if you’re in the middle of something.
https://techcommunity.microsoft.com/t5/microsoft-teams-blog/new-meeting-and-calling-experience-in-microsoft-teams/ba-p/1537581
https://microsoftteams.uservoice.com/forums/555103-public/suggestions/36883405-invite-entire-team-to-meeting