When you use the search functionality, you can use partial names or whatever you can remember. You just have to use ? for a character you don't know, or * for any number of characters you don't know.
Also, the "to" box is auto-completing - I can type "cai", wait a moment, and it'll finish "Cairnius"; if I type "ange" it will finish "Angelus Mortifer".
EDIT: If you really want you can just type "*" in the search box and get a list of every user's name, but it's not very well sorted.