There are two separate online systems to help you do this.
First, you look online at or for the current year: (2018 in this example)
(you can substitute any year instead of 2019 in that example).
Second, look at the online troggle-generated table of logbook entries and survey trips
(You need to scroll down past the table of attendees). This expedition report only shows those cave surveys which have been both typed up and entered into the online system, and also only those logbook entries which have been typed up already. So it will not be as up to date as the first method.
The most up to date method is to look in the lever-arch file containing the plastic survey wallets which is kept in the potato hut during expo.
The online wallet system and how it works are well documented in the survey handbook.
Third, and this is for when things seem to be going horribly wrong, look in the scans of the logbook, the bierbook and the callout book in e.g. in expofiles/writeups/2018 to find any record of trips which are otherwise unrecorded or lost and who was actually on expo on any one day. If even the scans don't exist then there is quite a bit of work to do before you can get on with the survey generation.