I installed the Domain Mapping plugin as per the instructions. Also configured the settings to specify "supporters only."
Domain mapping now show up in the Tools section on the main site, but I can't see it in the plugin area (either on site or in network admin) and I can't see how to specify WHICH levels should have access.
P.S. I am reading all the tutorials, but having a bit of trouble because they are often outdated, specifying menu options that don't exist and depricated names (like "supporter"), so maybe I missed something.