There doesn't seem to be a way to do that either. There are some workarounds described here:

https://moodle.org/mod/forum/discuss.php?d=142855

https://moodle.org/mod/forum/discuss.php?d=92958

Alternatively you could create a "User" report builder report, add the "time created" column and sort by that column to get the most recent signups.

Simon