You can search for another user by using the robust search feature. You can access the User & Topic Search by tapping the magnifying glass in the top right corner. Enter your search term and QuizUp will bring up the most relevant matches!
If you know the QuizUp username of the person you're looking for, you can always use the search tool. You can also use the search tool to find topics.
Start by tapping the QuizUp tab at the bottom of your screen. When you're in the QuizUp tab you should see the magnifying glass icon in the top right corner. Tap the icon and start your search.