Can you have an external Guest User as the sponsor?
Are they only able to enact changes based on be triggered with a review request, or can they initiate changes themselves?
Are the results of their Access Reviews immediately automatically enacted, or does this require review and implementation by a tenant admin?
Our other option is to code a control to provide this management, and use GraphAPI calls to make the membership changes. Would be very interested if anyone has any ideas on this, or if there are any other options we have overlooked to meet these requirements.