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If you are looking for a user from the Steam Community, you should try to enter his SteamID or a Steam Profile ID, ie:
  • http://steamcommunity.com/id/il_duce/
    (enter "il_duce")
  • http://steamcommunity.com/profiles/76561197996468970/
    (enter "76561197996468970")

If you are looking for a registered member, just type his/her username or display name, or parts of either of them.