With Microsoft Teams, you can chat one-to-one with someone or with a small group.
You start one-on-one and group chats the same way: by selecting New Chat at the top of your chat list.
Once in a chat, you’ll be able to send messages that include files, links, emojis, stickers, and GIFs.
One-on-one chat
Once you’ve selected New Chat and entered the person’s name in the To field, compose your message in the box and select Send. This starts the chat.
Group chat
Use a group chat when you need to talk to a small group of people. Start one the same way you start a one-on-one chat: select New Chat at the top of your chat list. Select the down arrow to the far right of the To field and type a name for the chat in the Group name field. Next, type the names of the people you’d like to add in the To field.
Once the chat has begun (whether group or one-on-one), click in the chat list to drop back into chat and send more messages.
Add people to the conversation
To loop more people into the conversation, click Add people 
(or View and add participants in group chat) in the top right corner of Teams. Then, type the names of the people you'd like to chat with, select how much of the chat history to include, and click Add.
Note: When adding someone to a group chat (not a one-on-one), you can include the chat history.
Teams will save 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.
See who's in a group chat
To see who’s in a group chat, hover over the number of participants in the chat header to show a list of everyone’s names.