Very interesting... question for you, can you add / view members in the newsletter module?
It seems that they use the same tables (site members and newsletter recipients), so could this be related to the issue that I've encountered here: http://www.silverstripe.com/site-builders-forum/flat/24702
EDIT: Just had a look at the security section, lo and behold no users are shown (but are in the database). I'd say that they're related.
MySQL version is 4.1.11
Running the query from the previous page (modded for my setup), I get...
SELECT `Group_Members`.`ID` IN (SELECT `ID` FROM `Group_Members` WHERE `GroupID`='4' GROUP BY `MemberID` HAVING MIN(`ID`))
#1109 - Unknown table 'Group_Members' in IN/ALL/ANY subquery
Let's see if we can do something about that...
EDIT 2: Yes indeed, it seems that HAVING MIN(`ID`) seems to be at fault. If it's removed, SilverStripe magically starts showing users again. I have no idea what repercussions that's going to have - hopefully none ;)
The next question then, is *why* that code was failing silently (silent fails are never a good thing...)