Include Skype users (not just Skype4Business/Lync users) as external.
I dont know if it is an option that we need to turn ON, but i do not seem to be able to add external users to a Team.
I would expect to be able to add any of the following:
* Email address (so messages can be emailed to a 'team').
* Skype (standard skype, NOT S4B / Lync)
* Skype 4 Business / Lync (apparently this is supported - but i just cant see it).