Security groups and roles can be a bit tricky to comprehend so I made a 3 part tutorial which covers it in small incremental steps and wanted to get your feedback if I forgot to cover something vital:
I just watched the videos, congratulations, I think they will surely help new (and not so new) users!
I edited your post to correct the link to the first video (it was the same link as the second video…). I think you need to go in your Youtube area and fix the video descriptions there, they have the same copy-paste “bug”.