Best way to work on 1 file with my team

I recently put a game dev team together, and we are almost ready to get a game going!
we want a way that we can all edit the same project file at one time, and then update the file
and have it “merge” all our work together.

so if I had a copy of our project, and the level designer had a copy, and we both imported models and made materials on our local file, could we then each upload our file to a host and merge our work together?

if you have an answer, PLEASE let me know!
our team is very eager to get to work on this game;)

I used dropbox, need to wait to synch files after someone from your team finished working and file size limit for basic plan without paying is something like 2GB, here are some links