If, in your site root, there is a directory 'sapphire', you're on 2.4 or below. If you have a directory 'framework' you're on 3.0 or above. On versions below 3.0 you should be able to see the version number in the footerbar of the CMS, to the right.
If this is an older site (say 2.3.x), this might have something to do with this ticket: http://open.silverstripe.org/ticket/3004.
[EDIT] Of course I don't know about your site and if this is at all possibl - is it in production already? - but I'd suggest you prevent other users from registering for the time being, until you find out what the cause is. Otherwise you might end up with a mix of password settings...