Anything that is physically manifested on this planet has a biofield.
Even rocks and minerals have an electro magnetic vibration permeating them.
A forest has a biofield which is the result of all the combinations of the individual biofields living in it.
This unified biofiled is the sum of all the trees, plants, animals, minerals, people and more present in it.
You can define biofields exactly the way you want.
For instance if you see two persons interacting with each other, you can describe the individual biofields and the way they interact with each other...
Or you can see these two persons as a new biofield entity and observe how this larger biofield interacts with other biofileds like other groups of people or the environment they are in.
http://vitalcoaching.com/biofield.htm