Earlier in May 2018, Microsoft has announced the availability of 1 year trial for Microsoft Teams, even if the user is not yet licensed for it.
This trial is now available to Office 365 commercial tenants (aka not Edu or Gov).
In addition of this Microsoft Teams 1 year trial, a new administrative settings is made available to manage how trial services (as of today only Teams is available but there could be more in the future).
To manage trials services, logon to your Office 365 administration portal (https://admin.microsoft.com/) with a global admin account and reach out the Services & Add-ins\User owned Apps and Services
Then you can turn on/off the option to Let users install trial apps and services (default is turn on)