Letting people subscribe to alerts through a form

I am trying to figure out how to create a form people can fill out in order to subscribe to topics on a list.

For example, a list of announcements is created with a "category" field which can include Athletics, Intramurals, Clubs, Activities, etc. A person should be able to fill out this form, and after submitting it be subscribed to alerts for the topics they chose.

Is this possible? I know there are ways to manually add people to groups for alerts, but is there a way to allow people to subscribe to their own alertsthrough the form?