How to Allow user to dynmacly create options in a checkbox field?

How to Allow user to dynmacly create options in a checkbox field?

I'm using customer portal and have many people log in and access form.  I want them to be able to set what the options are for a checkbox field.  Then when they come back to that form and use it later it will have their options in the checkbox field.

How would I do this?
Thanks
Scott