When you edit a record, you can select a user to assign the record to. If there is not a user record for the registrant, you'd have to create one manually in Joomla, Community Builder, or whatever you use.
Sounds like you just need to alter your setup. If you want all registrants connected to a login account... then enable auto user creation for your events and set it to required. Then if they are not logged in, they'll be forced to create a login. Then their registration records will be assigned to a login already.