As you know, Microsoft Teams is frequently evolving, adding new features.
That said, until now, you had not way to be able to evaluate these new features before they hit your tenant.
Well, you can now configure a policy to allow your end-users to have access to these incoming new features using the public preview program.
To manage this new capability and turn the public preview feature for your end-users (either all or a subset) connect to your Teams administration portal (https://admin.teams.microsoft.com/) and access the TeamsUpdate policies blade
There either turn on the Preview features setting on the default/global policy or create your own to target a subset of your users.
Then your end-users can turn on the public preview by access the AboutPublic preview menu from either Teams client or web.