Whogoes is supposed to be very trivial. It works in 3 steps:
Screen 1 : Type an event name
Screen 2 : Select the exact event
Screen 3 : Type a person name
That is it , you will see on the fly everyone who matches the name you typed.
The event must be visible to you and searchable. Not all events have this configuration. Whogoes completely relies on Facebook API. Sometimes, it responds lightning fast, sometimes not. Please bear in mind that very big events may take a while to iterate along all the interested/attending/declined list. For instance, an event with 100k people is taking between 3-5 minutes. Please be patient!