First thing to know, you must be a team owner to add a guest in Teams (see also Set guest permissions).
Sometimes you'll want to collaborate with people who aren't in your team’s organization. A lot of times you'll be doing that in email, but if you want to collaborate with them in Teams, you can add them as a guest. Guests have fewer capabilities than team members or team owners, but there's still a lot they can do. For a deeper look, see Guest capabilities in Teams.
Note: Before guests can join a team, an admin must enable guest access in Teams. If you can't add a guest, check with your admin.
Here's how to add a guest and work with them in Teams.
Add a guest to your team
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:
Select Teams and go to the team in your team list.
Select More options > Add member.
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.
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.
Click Add. Guests will receive a welcome email invitation that includes some information about joining Teams and what the guest experience is like.
Now that all that's done, the next thing you're going to do is set guest permissions.
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 your IT admin.
Identify guests on a team
To see if a team has any guests, just look below the heading where the team name shows up.
You can tell if someone is a guest by looking anywhere their name appears—the word “Guest” appears after it. You can also see everyone’s roles in the Members tab of a team. Go to the team name and select More options > Manage team and then Members.
Switch guest accounts in Teams
Go to the accounts menu to the left of a profile picture at the top of the app, then pick the team or guest account you want.
Team owners can set guest permissions, but only from the desktop or web app.