You might find it helpful to think if this as two separate agreements.
First a non-disclosure agreement should be signed by anyone you let see or use your confidential or trade secret information. This applies to your product in alpha.
Second, as Dana suggested, an Software Feedback or Software Evaluation agreement should be signed by any such evaluators IN ADDITION to an NDA. (Although the name is not terribly relevant as long as it is descriptive, I would not use "software test" because it has another meaning in the software industry.) The agreement should give you all the rights to their input, including the right to use, modify or abandon their suggestions.
I respectfully disagree with Dana that it can (or should be) for NO compensation. Seek the advice of any attorney in your jurisdiction, but it is a good rule to think of any agreement as requiring some "mutual exchange of value." As applied, this can be $10 or a user license for the software if it is launched. The point is that it just has to be something of value to the person signing the agreement. (It is often referred to in discussions as an "acorn" - or generic object of some value - especially of your testers are squirrels.)
Be specific, be clear, be honest, be fair, and don't try to slip anything in. Such contracts will often be judged against the company (you) and you don't want to accidentally give up some rights or ownership because you were trying to be "cute" in the agreement. Treat yourself and the other party fair, and you are on the right track.
And now for the standard admonitions:
Nothing in this information constitutes legal advice, nor does it create any fiduciary relationship including that of attorney-client. More generally this information was free and likely worth what you paid for it. :^)
If you are serious about this issue, have a CONTRACTS (transactional law) attorney write the agreement for you. For that few hundred dollars you may be protecting tens or hundreds of thousands of dollars of your work product.
