I'm surprised by all the people telling you to lawyer up; that's silly. If you want revenge, by all means, get a lawyer, get them kicked out of their accelerator, and make sure the venture goes nowhere. The odds of the venture being successful are so slim – if I were you, I'd sit back, relax, and wait to see if anything comes of it. If by some miracle its successful, you'll have plenty of negotiating leverage to bring them back to the table and find something fair.
There is some merit to this approach: continue to assert your IP ownership, but don't take legal action - just preserve the option to do so, if the venture takes off. However, you'd still want to talk to a lawyer to do this right.
Keep in mind that while shopping for a lawyer, you get essentially 30-60 minutes of free consultation (by phone or in person). So prepare a compact summary of the situation (documents and timeline) and then talk to a lot! (You'd be amazed how different their opinions are, so the sum-of-many is worthwhile.)
Consulting a lawyer has nothing to do with revenge. It doesn't even really have anything to do with taking legal action! A consultation is just that: getting advice. A lawyer will answer the exact questions the OP asked of HN -- usually for free -- and with the benefit of knowing what they're talking about. Talking to a lawyer does not necessitate any further legal action. It's about knowing what your options are and what the consequences before you do something. And "sitting back and doing nothing" counts as doing something too, and it may have consequences. Why wouldn't you want to find out for sure?
No offense, but have you ever actually talked to a lawyer about a civil matter? I've personally never had a lawyer pressure me to a file a lawsuit or take any aggressive action -- quite the opposite.
Edit: I see one fellow saying to have a lawyer send a C&D or some other sort of nastygram. I agree that's premature, although if he asked a lawyer to do that they'd most likely inform him about other options.
On first read, I think this is very, very bad advice. It depends on what you mean by "lawyering up," but all I see are other commenters telling the OP to talk to a lawyer.
He should 100%, absolutely, without question talk to a lawyer — if for no other reason than to understand his options and their legal consequences.
At an absolute minimum, he should have an attorney read over the invention assignment agreement. Beyond that, he has three choices WRT the agreement: sign it as is, sign a modified version of it, or sign no version of it at all. A lawyer is the only person who can honestly talk to the OP through the legal consequences of each choice.
For example, if he doesn't sign, what rights do the other founders have? What rights could they reasonably claim? Is he liable for anything? If he decides to use some or all of the software or other assets he developed while working on this project with these other people, could they have any kind of claim against him?
This is basic diligence and doesn't mean he needs to be aggressive, sabotage the project, get them kicked out of the accelerator, or anything else.
