Coexistence and interoperability between Teams and Skype for Business is determined by TeamsUpgrade modes. Everyone using Skype for Business is assigned a TeamsUpgrade mode, either by default or explicitly by their admin. If they've been assigned a coexistence mode, they'll be able to receive messages you send them from within Teams. Just search for them in Teams, type your message, and send it on its way. You'll see a Skype for Business icon next to the chat in your chat list.
For more info about TeamsUpgrade modes: https://docs.microsoft.com/en-us/microsoftteams/coexistence-chat-calls-presence
Group chat with Skype for Business users
Unfortunately, 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 pretty cool, but you’ll be missing out on fun things like emoji, file attachments, and tabs.
Turn off Skype for Business
You can't totally shut down the integration with Skype for Business. If you want to turn off the chat option, though, talk to your IT admin.
Note: If your IT admin disables 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 now do that from the mobile app. And while that's pretty cool, keep in mind that you'll be missing out on fun things like emoji, file attachments, and tabs.
You can't have a group chat with people who are using Skype for Business just yet. We're working on it.