How free Teams to communicate with 365 Teams

hongkongpost  - 

If your organization has Teams, the people you add will also need to be licensed.  If you wish to add guests to your teams, you can do that by adding individuals as guests (they may or may not have free Teams and it doesn't matter in this case) to your tenant/teams.

Within your own tenant, though, you can not have a mixture of paid and free users.  If they don't have a license, they need to be in as a guest.  Here's some more documentation that spells out in more detail.  I hope this helps: 

https://docs.microsoft.com/en-us/MicrosoftTeams/enable-features-office-365

https://docs.microsoft.com/en-us/MicrosoftTeams/communicate-with-users-from-other-organizations

https://support.microsoft.com/en-us/office/upgrade-from-teams-free-to-teams-29475bbd-a34f-4175-9b33-d44430f8ad39?ui=en-us&rs=en-us&ad=us