Newly created users are added to the role named "Authenticated Users"
Just now I implemented a new web.config setting that will be available after the next release of mojoPortal
<add key="DefaultRolesForNewUsers" value="" />
You will be able to populate that with a semi colon separated string of role names. If those roles exist in the site then new users will be added to them automatically.
However, I must say that I fail to see the benefit of being able to do this since the Authenticated Users role already exists I do not see what using a custom role there would provide any extra value since it is given to all users. If you want to add user to roles based on conditional logic you would have to do it using a custom user registered handler.
A more useful scenario is to grant paid access to a custom role that is used to protect premium content like in Site Membership Pro, and just use the built in "Authenticated Users" role for any content that should be visible to all registered users.
Hope that helps,