Setting up external users to use Teams
To invite users external to the organization there are several steps.
First you must add that person through their email to a Team - the External Users team is for that purpose.
To do this click on the elipsis next to the External Users team and click "Add member." Enter their email and select "Add xxx as a guest" as shown below then click the Add button and you're good.
Once they are a team member you can add them to chats, calendar entries, etc.
There are some steps that they will be required to go through to access Teams through authentication to a Microsoft Live Account.
These steps can be found at: