Hello,
A few people can work on the same project as long as they are doing it with plugins.
You can open one account for the team, and everyone could load and save games and plugins in that account.
There should be a master project that loads all the plugins.
Whenever you want to test the game, just update the relevant plugins and it will pull the code from the one who did it.
You can read more about plugins in here:
wiki.gamemaker3d.com/editor:blocks:actions:plugin
Regards,
Cyberix3D team