Congratulations Planner team on making this first important inroad into shifting mental models away from traditional organisational hierarchies to network-based team, and for granting outside team members access to Planner. This is an important first step as organisations are strategically making choices about what capabilities they will and will not develop themselves, leading them to choose to access specialist capabilities that aren't core to the organisation but which are essential to project success, externally.
Because teams are more and more composed of people from outside traditional organisational boundaries I suggest you continue evolving your mental models, and perhaps the language you use in thinking about teams.
People from outside the organisation might very well be guests from one perspective - let's call that mental model 'a' - but they are fully fledged members from a project perspective - mental model 'b'. As Renato Pereira notes, granting them access to Planner is essential (mental model 'b') but limiting their access by specifically not giving a project owner capability to assign the same permissions as they are able to assign to an internal team member is really a decision that appears based on mental model 'a'.
To help with the evolution of you mental models, perhaps you could drop the nomenclature 'guest' and instead call all members 'members'. If it is an imperative from a technology perspective to distinguish internal from external members after they have been granted access to the project, then a symbolic attribute might prove to be just as effective.
I have created a suggestion on UserVoice https://planner.uservoice.com/forums/330525-microsoft-planner-feedback-forum/suggestions/34180582-allow-project-owners-to-assign-the-same-persmissio.