Teams sites give you the benefit of adding external users to collaborate and share files with. In order to add users outside the organization (external users) the process needs to be done through Outlook online.
- Go to https://outlook.office.com/
- Navigate to your group on the left
- Click on the three dots and select Settings
- On the right, click on Edit group
- Click on the Members tab and select Add members
- Type in the email address of the user you want to add then select the user
- Then click Add
- The user should then get an email confirmation that they've been added to the group as a guest.
If you run into any issues, please submit a support request at https://techsupport.wileyx.com/