There are some good comments above, and I would simply add a few unstructured tid bits as well.
1) In the past, I have personally owned IP prior to the formation of a corporate entity that I then transfered that IP into the corporation as consideration for the stock I received as a founder.
2) Our council recently reminded me to be aware of the laws governing "minimum wage" - meaning that you should be certain to pay people at least that for any work done. In my case, we had equity-only agreements. We added a small cash component to satisfy the laws for minimum wage.
3) In your scenario, instead of retro-active documents, I personally prefer an "Exit Agreement" which is straightforward, plain english and clearly assigns IP, NDA, etc. for consideration either previously provided or provided at the time of signing.
I will now provided all the usual disclaimers that I am not a lawyer, and all this is simply my humble opinion.