Hi skug89,
If it is very long hair or a pony tail, you can have it as a separate symbol on the character timeline attached to the head and placed on a layer behind it.
A similar question was asked a few years ago and then I quickly created this rig as an example. It should give you an idea how to approch the problem.
Depending on your design and desired behavior you may also experiment with soft-welding the hair.
Hope this helps!