IANAL, but I have spoken to lawyers, and from what I gathered, if you want to be funded at some point it is important that you tidy up the intellectual property

IANAL, but I have spoken to lawyers, and from what I gathered, if you want to be funded at some point it is important that you tidy up the intellectual property situation. You probably just need a paper that says that whoever's leaving did own the IP put into the start-up and that they're transferring all their rights to the company. Investors like to know that you own what you're selling...