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 TeamDavid Attenborough commented
Adding support for this request. Please, please, please make it high priority:) Our business loves Planner but being a Consulting business we want to share with our partners and customers alike!!!