@margaretdodge unfortunately, that's how Planner was designed and the Dev team has made zero indications that they're planning to change this anytime soon
- You must be a M365 Group member to view and edit a board
- All group members have access to ALL plans within the group; you cannot limit access on a user-by-user basis
- There are virtually no data-loss prevention tools within Planner; that means if someone edits or deletes tasks, lists or entire boards - you are SOL.
My recommendation is to look at Microsoft Lists to manage your projects if you need to manage permissions. Either that or a 3rd-party tool.
Sorry to be the bearer of bad news. I wish Planner was better than this but it simply isn't and quite frankly, I'm starting to think it never will be.