As an update, we are in the testing phase of implementing guest access. We are also ensuring that guest access will be supported when using Planner within Teams!
We will update this thread again as soon as it’s available for all users.
The Planner TeamAnonymous commented
Sigh...another feature I look at and think "great, that'll help me work with my consultancy clients / partners" then find out it's barely out of Alpha and doesn't support external access. Same story with teams. In both cases requests made years ago and continual missed promised deadlines or no response at all. Microsoft seems to be flogging vapourware more and more nowadays.. I'd ask what the status of this is but since it is marked as "started" well over a year and a half ago and no update since I'm not anticipating a reply.