When attempting to share a dashboard, users with supervisor rights are not on the list of groups to share the dashboard with. There is no group assigned or that can be assigned to users that have supervisor rights.
Currently, there is not an option to share dashboards with users that have supervisor rights. The workaround for this would be to create a security group that has all rights in the database that mimics the rights of users that have supervisor rights, and then assign that to the users with supervisor rights in order to share the dashboard. If you would like to have this functionality be considered by our development team to be added to Raiser's Edge in a future release, please submit the idea to our idea bank: https://community.blackbaud.com/products/raisersedge7/ideas