Hi Mohan,
Do you mean Guest Portal? The Sponsor Portal is for users with Admin-rights to congiure guest accounts.
If Guest Portal this might be what you're after...Take a look under:
Guest Access -> Configure -> 'Default Portal' (yours maybe called something else)
Under the heading 'Login Page Settings', enable the checkbox:
'Allow guests to create their own accounts'
Now when they arrive at the Guest Portal there will be a link 'Don't have an account' to allow user self-registration.
cheers,
Seb.