External users can only be added within the Teams Channel - and this is successful if the external user have a Microsoft Account. If the external user does not have a Microsoft Account yet, he/she may create one thru this link:
NOTE: ONLY the OWNERS of Teams Channel can ADD a member - including an external user. MEMBERS on the other hand, can only add an internal user via a request with the Owner's approval.
To add an external user, please follow the steps below:
1. Click the Teams found at the left side of your MS Teams screen.
2. Select which channel you are intending the external user to be added.
3. Once your selected channel have opened, please click the ellipses to display the menu options.
4. Click Manage team.
5. A new window will open, click the Add member button.
6. A pop-up window will open. Type the external user's email address (please take note that the external user's email address MUST be linked to his/her Microsoft account). A suggestion box will appear below, click Add <external user's email> as guest.
7. Then finally, click the Add button. Then click Close.
https://competenz.freshservice.com/support/solutions/articles/5000069921