Community of Users
In order to view a Plan you must be able part of the Group which the Plan is aligned to.
If the Plan has been associated to an Event then in order to see the Plan Users must be part of the Plan Group and all Event Groups.
In order to make edits to High Level Plans you must be part of the Plans Community you must be added to the Community you must be of Author permission level and above and part of the plans group.
From the ... More Options menu navigate to Community.
The Community Modal window will be launched from here you cab maintain the Users who are part of the Community. As the creator of the Plan you are automatically added to the Community. Potential Members which can be added will be listed under Potential Members.