If a person is not in invisible/hidden mode and they are on your Buddy list, you will be able to see their name in bold in the list of those online at the bottom of the screen. If you have set profile settings (like IM info) to Buddies only, only those you add to your list will be able to see IM contact info you input.
As for the Ignore option, it is possible to disable receiving PMs from those on your Ignore list. In addition, adding someone to your Ignore list will "hide" their posts on the forum so you don't have to read them, but you maintain the option to choose to look at individual posts they make.
Either way, no one is notified if added to either list to my knowledge. The only way those on your Ignore List might know if you've added them to it will be if they try to send you a PM. I believe they will receive a message about how they can't do that.