I am 18, my girlfriend is 16.

Do I need her parents permission or do they even need knowledge of our copulations?

In Michigan, the age of consent is 16. This means that an 18 year old could not be charged with violating a criminal sex offense law if they were having consensual sex; however, being that the 16 year old is still a minor under the law, her parents have primary legal authority and control over their minor child and are legally accountable for the childs welfare.

If the parents of the minor do not consent to their child's relationship with the 18 year old, it is within their parental authority to take legal action to prevent the 18 year old from contacting their minor child if it becomes necessary.

How you communicate with her parents is really up to you. Because you are a legal adult (18) and the 16 year old is still a minor child under the law, you are essentially taking responsibility for the welfare of a minor when she is in your company and away from her parents. Her parents have a right to know who they are entrusting with this responsibility and further, they have a legal right to say NO.