Inviting users in highrise - random url

I'm trying to figure out how highrise invites users. When inviting a
user, the following url is created:

highrisehq.com/users/new/e586dbbccad1

Where is that random string being stored? Is it a custom resource
method?

I would like my users to be able to set up their own account, rather
than my app creating an account and asking them to login. Can anyone
explain to me how this is done? Thanks.