In my previous experience with jams, it should be fine to just credit every person who contributed over the entire course of your project.
I've went down to this jam's FAQ and here's what it says about group project policy:
Q: Are groups allowed?
A: We strongly encourage the creation of groups to meet the deadline! To participate as a group, at least one account needs to press the Join button and share your entry. To make your entry stand out, remember to personalize this account with your members list and profile.
To me, that reads like your situation should be *fine*. Your last-to-the-table member just won't get email notifications about the Jam. I think that in the process of publishing your submission you can still add their itch.io profile to the list of contributors, along with other team members: look out for that option.