If external users are not configured already (you don't see a sw-External-Users in webadmin (left side in the tree view) then a external directory has to be installed like OpenDS and configured in the 8090 console first.

If it is already configured, then you go to the External Users panel in webadmin and assign the external users to an object like a file node there. Open a object like a file node and then from the drop down choose External users (of just keep clicking NEXT to change panels)

Because they are not part of main directory, they have to be assigned slightly different than normal users.