Group chats in Teams can now include external people.  It’s as simple as creating any group chat in Teams.  First, just make sure you can message the external people to be included in the group.  In other words send them a Teams chat message.  Then simply create a new group or add them to an existing group.

Start a New Chat

Click the new chat icon.

Enter the name of the person. 
To send a message to someone in a different organisation use their full email address rather than just their name.

Compose your message

Click Send

Start a New Group Chat

Click the new chat icon

Enter the name of the first person, enter names of other people

Compose your message, use

 to send a longer message.

Click Send

Add People to Chat

In the chat, click

Enter the name or email of person to be added

Choose the amount of chat history to be shared with new members

Click Add

Rename a Group Chat

Select the group chat

Next to names click pencil icon

Enter new name

Click Save