I've been messing with a thing called Group for the WYSIWYG that should do what you need. I've found it to be a little balky and frustrating but I am able to eventually get it to do what I want.
Go to Admin/Wysiwyg and at the top of the WYSIWYG config page you'll see a button that says Toolbars. Click that and on the toolbar page look in the Plugin Tools area for "Group" Drag the icon onto the toolbar.and save.
Now when editing, type the text you want to hide, highlight it all and click the Group button. Fill in the allowed groups by name separated by a pipe (I always have to copy it from the instructions because I can never remember how to get a pipe.)
Now that part of the page is seen only by the allowed groups (or hidden from restricted groups). So two users, one in a permitted group and one forbidden can read the same page at the same time and see something completely different!