Yes, you need to have a WRITTEN agreement that does at least the following, for starters:
first, a work for hire agreement and copyright assignment, which should be prepared by an IP attorney ;
Second, a secrecy and non disclosure agreement as to your business plans and customer lists,
Third, an invention assignment clause so you get any inventions they create for you -this should be written by an IP attorney.
Fourth, the waiver of any compensation and recitation of the experience as full consideration.
There will be many other understanding that need to be agreed at the start, before creative work begins, especially what the worker must do and must not do, and be very cautious about post departure restrictions, such as any non-competition clauses.
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I would if I were you. Recently, the Huffington Post was sued by several of its bloggers because they now want renumeration. Consult a local business attorney well versed in internet/cyber law to draw up a contract. Use the "find a lawyer" tool on this site.
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YES! You need an agreement regarding who keeps the copyright of the posted blogs, and what the expectations are for both sides. Are they indemnifying you if they plagiarize from some other source and you get sued? Many issues here.
This is basic training for the young people you are working with, as well as you.
I'm not your attorney; my answer includes assumptions. If you want me to be your attorney, I'm easy to find.