Add or invite people outside your org to a chat in Microsoft Teams

With External Access for Teams, you can add or invite anyone outside of your Teams org who uses Teams, Teams for personal use, Skype (for consumers), and Skype for Business to a one-to-one chat. 

For those who use Teams or Teams for personal use, this includes both one-to-one chats and new or existing group chats.

First things to know

• If you invite someone who doesn’t have a Teams account, they’ll receive an invite to start an unmanaged Teams account and then join the chat with all chat history included.
• People from different Teams orgs can be easily added to each other’s chats without sending invites.
• People who use Teams for personal use must accept a chat invite to participate in a chat. When a chat is created between a managed Teams org and a Teams for personal use account
• External Access is turned on by default, but it can be turned off or restricted by an org’s admin, such as blocking certain addresses or domains.
• An external label at the top of a chat indicates if there are any external participants in the chat. You’ll also see an identifying label beside the names of any external participants in one-to-one and group chat participant lists. On the upper-left side of Teams
• A person’s status indicator is always shown for people who use Teams and Skype for Business, only shown for people who use Teams for personal use after they accept a chat invite, and not shown for people who use Skype.

Add or invite someone from outside your org to a one-on-one chat

People you add or invite outside of your org to a one-to-one chat may include anyone who uses Teams, Teams for personal use, Skype, and Skype for Business. 1.On the left side of Teams, select Chat and then New chat to start a new chat.

2.Type the person’s email addressor phone number that you want to invite to chat. 

If their name appears, select it from below the command bar. If there isn’t a name match, select Search [email address] externally to send them a chat invite. 

To start the chat, type a message in the text box at the bottom and then select Send .

Add someone who uses Teams outside of your org to a group chat

People you add who are outside of your org to a new or existing group chat must use Teams or Teams for personal use. 
Start a new group chat and add participants

1. Type the email addressesor phone numbers of the people you want to invite to chat. 
2. Type a message in the text box at the bottom or click the down arrow on the right to add a group name (optional) before typing a message.

Add participants to an existing group chat

You may only add external access participants to existing or ongoing chats that already include at least one external access participant. Otherwise, you’ll create a new group chat with the internal participants and the one or more external participants that you’re adding.

1. Choose or be active in the existing group chat that you need to add the participants to.
2. Select View and add participants below your profile picture at the upper-right of Teams, and then choose Add people.
3. Select the added participant’s name, then choose if and how much of the chat history you’d like to include. Select Add. 

You’ll then see those people added to the group chat.

If there isn’t an immediate name match for a person (no results found), choose Search [email address of person] externally to send them a chat invite.

4.To start or continue the chat, type a message in the text box at the bottom and then select Send.

Credit : https://www.microsoft.com/