New Microsoft Teams is just AWESOME, quick but useful post below shows how you have this preview feature to make your life EASY!
Open Microsoft Teams admin center [Ask admin in your organization if you don’t have access] and follow path Teams > Teams update policies > Click on an existing policy or Create New > follow step 1 and step 2 below.
Toggle Try the new teams to get yourself into NEW TEAMS world, you may chose to go back to classic (GA) version at any point of time.
Let the change happen!
Get it, Preview new Teams!
Experience teams across tenants and get notifications from multiple tenants accounts. This is PRETTY COOL!