@mention people from outside the team grant them access to that conversation only on private teams
Just like Yammer that you can @mention anyone granting them access just to that conversation or Planner task without adding them to the group or team or channel.
Zack Sloane commented
This needs to be a feature. We need the ability to "ask questions" of people outside the team. There are many cases when some in HR, Facilities, Health and Safety, Legal, etc etc might need to be brought into a specific conversation within a team. It wouldn't be desirable or even manageable to add those people to the team in order to ask them a simple question (with the context of the conversation there for them to read)