The member list in the Wikidot admin panel is painful for site sizes over a few hundred. It does not permit you to go more than a couple pages at a time, and there is no search method to find a particular user, you must instead linearly crawl until you find the user in question.
However, you can directly call WIKIDOT.modules.ManageSiteMembersListModule.listeners.loadMemberList(<pageNumber>) to jump to a particular page, so it actually is possible to automate this as a binary search instead.
This is what this Greasemonkey script does. Enter the date the user joined the site on (not when they joined Wikidot! Also, remember to get the join date for the correct site) and hit search. The preferred date format is DD Month YYYY, the same format used by Wikidot in its user info modal.
You can install the script here.
Available under the MIT License.