Sharing an account does not mean your share the project. No project files are syncronised with Epic and your project only lives on your computer. If you want to share it with someone (regardless of if / how they are logged into Epic), you must share the project files by copying them to the other persons machine.
Copying the files manually by usb / network / whatever is a pain because it’s very hard to detect conflicts (where two people have made a change and you’re not sure which file you now want). This is why source control is a good idea (amongst other reasons). I strongly suggest you look into setting up a Git / Perforce repo to share your project. It will let each of you edit the entire project and whatever other assets you have, then you simply commit those files so they can be shared amongst your team. Changes to the files are tracked so you can recover earlier versions in case of emergency, and if there are conflicts, you are made aware of this and can choose how to resolve it.