some sort of reference no. with wp-member plugin?

All users in the wp_users table have a numeric primary key assigned, this is the ID field. However, if you want something besides 1,2,3,etc, then you could generate your own unique number or reference (it need not be numeric).

After the user registers, there is an action hook wpmem_post_register_data. At that point, the user is registered and the ID is available to you, so you could hook a custom function to that action, create your unique reference code, and save it as a meta field (using https://codex.www.ads-software.com/Function_Reference/update_user_meta).

where can I hook the code for reference no., I am not that much fluent in php.

And one more thing can I have radio buttons for registration forms?