1. Run, don't walk, to the copyright office and register. It costs $35, you can do it online, and if you do it in time it puts you in a better negotiating position. I'm not going to walk through the legal situation here, just do it. Your lawyer will thank you.
2. If you think owning the IP was a provision of the accelerator deal, you could probably brick the deal by writing to the accelerator yourself and pointing out that you own the IP. I wouldn't do that without a lawyer's ok, but I would definitely make a threat as one software engineer to another to the founders that it's within your power to contact the accelerator and see if the cofounders blink.
3. Is the IP being used by them right now? And how is it being used? There are potentially some strategies there but I would need more details. If you don't want to share that information here then lawyer up.
As Toby Keith quoted to me one day: "A word for a third". Copyright is implied at the time of authorship and is legally enforceable regardless of registration. Registering the copyright lays down a legal argument for your ownership, but it doesn't preclude other's ownership of the project. Op said "with a large group, including only one other dev", which means there are potentially others involved as well. I agree with your #1 suggestion and would add the application should include the names of all involved.
Before getting the attorney involved in communicating with the other parties, I'd suggest the Op mention to the other parties his copyright to the code/name/project is implied and he's filed for copyrights on the project for all involved. This gives the other parties something actionable, without getting too aggressive. They may or may not choose to do the right thing at that point.
Unless this is a work of content or some kind of content (e.g. a logo design?) is integral to the startup, copyright is not the primarily relevant IP doctrine here.
IANAL, so disclaiming any sort of legal advice here, but the contributions a founder would make in helping to collaboratively create a new company -- everything from a proprietary idea to business model to naming to market entry strategy to logo to features and functions to contributions to code -- go far beyond copyright. With circumstantial evidence pointing to collaborative creation, and no agreement to the contrary of joint ownership (e.g. a "work for hire" contract) or any agreement specifying specific proportions of ownership, a court would be reasonably likely to rule that OP is a partial owner of the entire company. (Whether it is worth the cost of winning this claim is another matter entirely.)
Whereas copyright is the more limited right to own a specific expression (e.g. writing, images, code syntax) in its exact or near-exact form.
Here is the form of agreement you might sign during the formation process if you decided, after a startup weekend, to jointly form a company:
"Developer hereby assigns to the Company exclusively throughout the world all right, title and interest (whether or not now existing) in (i) the subject matter referred to in Exhibit A (“Technology”), (ii) all precursors, portions and work in progress with respect thereto and all inventions, works of authorship, mask works, technology, information, know-how, materials and tools relating thereto or to the development, production, use, support or maintenance thereof and (iii) all copyrights, patent rights, trade secret rights, trademark rights, mask works rights, sui generis database rights and other intellectual property rights and all business, contract rights and goodwill in, incorporated or embodied in, used to develop or produce or use, or related to any of the foregoing ((i), (ii) and (iii) are collectively “Intellectual Property”)."
As you can see it includes copyright but is a superset.
>> not the primarily relevant IP doctrine here
Judging from a quick read of his other comments I'm thinking that he might be saying that there is another aspect of IP law that would cover it. I initially felt the wtf-ness as well. It might be nice for him to elaborate on which would be the relevant doctrine.
1. Run, don't walk, to the copyright office and register. It costs $35, you can do it online, and if you do it in time it puts you in a better negotiating position. I'm not going to walk through the legal situation here, just do it. Your lawyer will thank you.
2. If you think owning the IP was a provision of the accelerator deal, you could probably brick the deal by writing to the accelerator yourself and pointing out that you own the IP. I wouldn't do that without a lawyer's ok, but I would definitely make a threat as one software engineer to another to the founders that it's within your power to contact the accelerator and see if the cofounders blink.
3. Is the IP being used by them right now? And how is it being used? There are potentially some strategies there but I would need more details. If you don't want to share that information here then lawyer up.