Allowing External Users to Edit Success Plan Information

We would like to allow external users to edit Success Plan information. We have plans created around client project and now have several clients who wish to edit the plan information independently such as updating the project team, goals, success metrics, resources, etc. We currently have one client refusing to use success plans due to the limitation of not being able to edit the plan information. This would allow our clients to take a more active role in their success plans generating ownership and buy in.