When you are in a meeting, you are able to chat with anybody who is present in the meeting. Select the speech bubble icon in the upper left to open the chat. Messages you send will be visible to everybody in the meeting, and they will also be posted as replies to the meeting notification in the General channel of the team where the meeting originated.

Teams provides a chat app that exists outside of any specific Team. Open this app by selecting the Chat icon in the left-hand menu. Select the new chat option to begin a conversation with any of your contacts or anybody in your organization. You can create individual or group conversations.

Chats within the chat app will remain accessible in this persistent space, allowing you to quickly refer back to any conversation with a specific individual or group. You are also able to share files in this chat and initiate audio and video calls.

If you would like to send a message to everybody in a specific team or channel, you can enter it into the Posts tab. Simply select "Start a new conversation" at the bottom then type your message. You can include @tags to make sure specific people or groups are notified of your message, but everybody on the relevant channel will be able to see your message.