How do you find people you know on ask?
You don't. Most people on Ask do not use their real names. And this isn't really a social chat site. Most people that I know do not use ask.com for leisure and enjoyment.
... search for their username on Google ...
You can't search a name. You have to find a question they asked, or an answer they gave.
You can't but you can ask them if they have an ask.com account
No way to find people here. You can find certain people at http://goo.gl/TYa7sG.