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 TeamJustin Hitt commented
Modern businesses leverage outside users. If I'm going to move my business into Office365 it needs to let me bridge internal accounts as well as work with clients and partners without special logins. Tracking vendors in Planner won't work without third party assignments.