Coexistence and interoperability between Teams and Skype for Business are determined by TeamsUpgrade modes. Everyone using Skype for Business is assigned a TeamsUpgrade mode, either by default or by their admin.
If someone has been assigned a coexistence mode, they'll be able to receive messages you send them from within Teams. Search for them in Teams, type your message, and then send it. You'll see a Skype for Business icon next to the chat in your chat list.
For more info about TeamsUpgrade modes, see Coexistence with Skype for Business.
Group chat with Skype for Business users
Group messages can’t be sent to people using Skype for Business. However, whenever someone invites you to a group conversation from Skype for Business, you’ll get a notification in Teams. You'll then be able to join the conversation back in Skype for Business.
Skype for Business vs. Teams
Being able to chat with someone on Skype for Business from within Teams is great, but you’ll miss out on fun things like emoji, file attachments, and tabs.
Turn off Skype for Business
You can't completely turn off the integration with Skype for Business. However, if you want to turn off the chat option, talk to your IT admin.
Note: If your IT admin turns off chat, you'll still see all your Skype for Business meetings but you won't be able to chat with Skype for Business users.
If you want to chat one-on-one with someone who's using Skype for Business, you can do that from the mobile app. And while that's great, keep in mind that you'll be missing out on fun things like emoji, file attachments, and tabs.
Creating group chats with people who use Skype for Business isn't available yet.