I actually did this by using workflow (as you guessed).Since users are content object you can trigger an approval workflow before publish.
It gets a bit tricky when dealing with validation emails since these emails are sent to the user which validate its account in turn but in the mean time your administrator has not yet approved the new user.
You might end up with a situation where the user believe that he can connect onto the site though he actually can't because he is not (yet) approved.
To work around this I changed the text of the sent email to explain the user that he must me approved before geting access to the site. I also used a module(*) to send an email to the user when he is approved.
Hope this helps - If you (or anyone else !) find a better way to achieve this, please let me know.
rgds
Olivier Pierret
(*) Mailinglist (Workflow Event) in contribs/workflow section
Hello. When I apply a pre-publish Approve/Event workflow trigger to the "Users" section, I would expect that this means that new registrations would need to be approved by my Admin user. However, the workflow trigger never fires. New users can register without going through workflow at all.
I actually did this by using workflow (as you guessed).Since users are content object you can trigger an approval workflow before publish.