Create a HOOK to be informed when someone enroll or start to unit

Hi there,

I would like to use an option to send an email if someone enrolled to a free course (or start to a specific unit) with an automated email plugin such as MyMail. There is an option to set to send an email with a hook. Is it possible to define a hook that point if someone enroll to a course? If it works, I'll set a hook to send a specific email to new subscriber.

Thank you,
Orhan