Right now when user needs to add someone to it's own pool he types username in text input field. This value is then submitted to server and is written to the 'username' var. After that 'addPoolUser' local function validates submitted value and searches corresponding User entity in DB.
I suggest to replace this text input field with Lift's AutoComplete widget, so when user begins to type text, SQL query like "select nickname from users where nickname like 'typed_text%'" is executed via ajax call and list of matched nicknames is displayed. User subsequently selects nickname from list and it is then saved into the same variable 'username'.