Guest access is live in Planner! To learn more, visit: https://techcommunity.microsoft.com/t5/Planner-Blog/Bring-your-plans-to-life-with-guests-in-Planner/ba-p/190704
We have two more parts of this feature to deliver:
1) Comments enabled for guests to see and use
2) The ability to invite a guest from within Planner
Thank you again for your patience and feedback on this!
The Planner Team
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Could this improvement be merged with syncing tasks across multiple planners? A separate Planner for external and/or other internal teams for which you can share and sync specific tasks or the entire plan.