As a team owner, you can invite guests to your team. Just go to the team name, select More options > Add members, and enter the guest's email address.
Guests need a valid email address.
If you receive a "We didn't find any matches" error while trying to add a guest, your organization doesn't allow guests. Contact your IT admin for more info.
Next, add your guest's name. Select Edit guest information and type their name. Be sure to take the time to do this now—you’ll need the help of an IT admin to do 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.