To find usernames you must have administrative privileges within Sifter. As an administrator you can click the People & Organizations link at the top of any page in Sifter, make sure you're in the People tab, and you'll be able to view a list of the users on the account. To find someone's username just click their name in the list and it will take you to their profile.
There is no way to view a list of everyone's usernames at once.
If you need the a user's username for mentions, simply start by typing "@" and their name it should find the user and autocomplete for you.