To answer the question in your comment, yes, there is a difference in the search parameters when using the dashboard or top menu bar. It searches for specific tools and functions. As you type a keyword, it will auto assist with a drop down menu to help you pinpoint the tool or function you are looking for.
When you click on a tool that requires a username, as you type, it remembers previous usernames and will offer a drop down menu to help assist with locating the person. As mentioned in another answer, it is case sensitive and you have to avoid spaces unless the user name has one inside it.
To expand on my answer, PGC has a sophisticated search capability that you can use from the Dashboard https://project-gc.com/Home/Dashboard and paying members have Advanced Search capabilities that take it even further.