Teams - Add a Guest
Guests must have an Office 365 work or school account. If the guest doesn’t yet have a Microsoft account associated with their email address, they will be directed to create one for free.
To add a guest to your team in Teams:
Step 1: Select Teams 
and go to the team in your team list.
Step 2: Select More options 
> Add member.
Step 3: Enter the guest's email address. Anyone with a business or consumer email account, such as Outlook, Gmail, or others, can join your team as a guest.
Note
If you receive a "We didn't find any matches" error while trying to add a guest, your organization doesn't allow guests.
Step 4: Add your guest's name. Select Edit guest information 
and type a friendly name for them. Be sure to take the time to do this now—you’ll need the help of an IT admin to do it later.
Step 5: Click Add. Guests will receive a welcome email invitation that includes some information about joining Teams and what the guest experience is like.
Note
When you add a guest, only their name gets added to their profile card. To add or change other info (such as phone number or title), you'll need to contact the service desk.