Four things to know about group chats in Teams
There are a few different ways to chat with people in Teams:
In a one-on-one chat (so with one other person)
In a channel on the Posts tab (so with everyone on the team who pays attention to that channel)
In a group chat (so with a few people at once, but outside of a channel)
Here are four things to know about group chats:
1. Name your group chat so it's easy to keep track of.
Remember to name your group chat, so it's really clear what the subject of the conversation is.
To start and name a new group chat, select New chat 
at the top of your chat list. Then, select the down arrow next to the To field and enter a name in the Group name field. You'll see the name at the top of the group chat.
To rename a group chat, select Name group chat 
at any point.
Name your group chat so it's easy to track.
2. Pin your most frequently used group chats.
Pinning is a great way to keep track of a chat even if you only need to do it temporarily.
To pin a chat, highlight the chat, then select More options 
> Pin.
Once you pin a chat, drag it into the order you want. Do this for several chats and you'll have all pinned chats within easy reach.
Unpin the chat when you don't need it front and center anymore. There's a limit of 15 pinned chats.
Pin your group chats for easy access.
3. Share files and make calls right from your group chat.
In a group chat, you can share files and collaborate on them. Then, instantly make calls to the whole group of people who are in the chat. It's really handy for that!
Here are two ways to share files in chats:
Go to on the left side of Teams, highlight the file, choose More options 
> Copy link, and paste the link directly into a message.
In the toolbar under your message, click Attach 
, and upload your document.
The Files tab in your chat shows all the files you've shared with each other.
The Files tab in your chat shows all the files you've shared with each other.
Select Audio call 
to start a call from a group chat.
4. Select how much chat history to include.
When you need to add somebody to a group chat, you have a few choices about how much conversation history you can have.
To see your choices, select View and add participants 
in the top right corner of Teams, then selectAdd people. Type the names of the people you'd like to chat with. Next, select how much of the chat history to include, and click Add.
Teams saves the whole chat history, all the way back to the first message. And if someone leaves the group their chat responses are still there in your chat history.
When adding someone to an existing group chat (not a one-on-one), you can include the chat history.