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.
Note: Guests need a work or school account for Office 365.
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 head over to the desktop app to begin the process, though.