First of all, consider using some file versioning system, like Git, SVN or Perforce. With systems like that you will be able to keep your project stored in a secure manner and always synched with each of your developers.
Binary files cannot be merged, so it’s not so easy for two or more people work in the same map.
You can do some workarounds like the new Live Editor of Unreal, check out this doc:
Or you can split your map on different level streamings and make a great politic of file saving inside your team.