New group creation during user registration based on custom field

This is my requirement
1) Add a custom field to user registration form (I know I can do by adding a custom field from admin or modifying registering.tpl
2) If the value entered by user doesn't exist as group, then create a new group and assign to user.
3) if the value entered by user exist, then just assign user to this existing group.

Can somebody help me with file names and piece of code I need to modify.

This depends on what the custom field is and where you plan to store the information in the database. Users can have custom content preferences - so you might be able to store your data as a user preference. Or you may wish to use your own table to store the extra data.

Likely you'll have to write a service for users to store and lookup this data. To help you more we'll need to know more details about what you are interested in adding.