) You can come over to my place whenever you become free. Good enough
2) The teacher sent an email to each student with the instruction to attend the exam.
3) Receive an email to inform about housing.
This is wrong. Who or what receives/received the email. It's not possible to work out what you mean.
Currently it's in the imperative (command, order, request) form. But you can't order someone to receive an email.