Directing messaging an external client on Teams

[redmoir]

I currently host meetings with multiple users on Microsoft Teams. Some of these members have Microsoft accounts and as such come up as guests. I am able to easily direct message those people. Others, however, do not have Microsoft accounts and so I was hoping I could get some advice on how to direct message these individuals. I hover over their names and the chat button is unavailable. 

Thanks!

[#2]

In order to use the chat functionality, their account needs to be associated with Teams, Skype for Business or Skype subscription. Generic XMPP support/federation was deprecated a while back.