@Elliott404 you're wrong - and dropbox is the worst way to handle such a case.
@maxpain It is possible to work with several people on a project, but it is not easy because you need to do a manual synchronisation during that work.
1) one of you needs to hold the "master" copy of the project. NO ONE can work on that copy directly.
2) in the master copy, make a number of new maps (all empty). I suggest something like a dozen maps for everyone who will create maps for the project.
3) copy the master project and give each team member one such copy to work on. Assign map IDs to those who will create maps, something like "Tom will work on map ID 1-12, Anne will work on map ID 13-24" and so on. Additionally, assign some people to work on specific segments of the database like skills or enemies. Also assign switch and variable IDs to be used.
4) Now everyone can work on their parts, but it is important that no one works on parts he has not been assigned - if someone needs a change there, tell the person who works there to make the change.
5) at regular times (perhaps once per week or every other week or so) everyone stops working and sends his/her mapfiles and database files to the person who holds the master copy.
6) if nothing went wrong, there should be no duplicates on the files send to the master - all of those are simply copied back into the master project. If there is a double file, then something went wrong with the work assignments and you need to sort that out before deciding which of the conflicting files to use.
7) after all updated files are in the database, the master project gets opened and for a short time it will be worked on to create connections.
That means map connections outside the map groups are done, the maps are reordered in the map tree (if needed) and new empty maps are created and assigned (to give the mappers new places to work on).
8) go back to point 3 and repeat until the project is finished.
make sure to check the data files of a project to know which parts of the database can be worked on independently.
WARNING: part of point 7 is also to consolidate the names for switches and variables, that has to be done manually and can't be done by filecopy.
So yes, it is quite a bit of work and organisation to have several people combine to a team that way, but it is possible and has been done.