If you're a team owner, you can now invite guests to your team. Go to the team name, click More options > Add members, and then enter the guest's email address.
Guests just need a valid email address.
If you receive a "You are not authorized" error while trying to add a guest, your organization doesn't allow guests. Contact your IT admin for more info.
For guests, you need to add the name other people will see. Click Edit guest information and then type their name. Be sure to take the time to add their name now—you’ll need the help of an IT admin to add it later.
More about guests in Teams
Good news—you can now add guests to your team! You'll need to go to the desktop app to begin the process, though.