Step 1- Sign in to Microsoft Teams. On the left side is a list of the different teams inside your organization.

Step 2- Right-click the name of the team you want to add a guest to. In the menu that pops up, select Add Members.

Step 3- Type the email address of the person you wish to add. If outside your organization, Teams will automatically add them as a guest.

Wait for the user to accept the access request, and they’ll be able to log in and use Teams just like a standard user.

Recommended reads:

Dealing with slow/reduced Bandwidth when using Microsoft Teams