How to make a custom form?

Hi. I have installed your plugin and the plugins looks very close to what I have been looking for. So its great that I have found it.

I need to know few things about it.
1- Is there any way I can customize the form? I want to append some new input (mostly text and drop-down) fields. How can I do that?
2- The plugin offers to create groups. That is nice. It would be great if by some means, I get the ability to ask the users directly to subscribe for a group? e.g, by some means, would it be possible that I list all of my groups on the plugin’s form (output) and then based on the selection by the user, he get subscribed to that specific group he chooses?

Thank you in advance.

https://wordpress.org/plugins/email-subscribers/