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