a group of people have to modify the same file. there are some problems.
1. Is there a way I could view who has modified this file in a particular time.
No. Please note it is an online storage/sharing/collaboration system, not a version control system.
2. when user John is modifing the document, can he lock the file? Because i don't want anybody else to modify the document at the same time. I want them to modify a document one by one. In that way, I can trace who did well in document and who didn't.
It is not recommended to lock a file over HTTP. There is no easy way to track if the user has finished working on a file. A user could lock a file; if his computer crashes or network is disconnected, the server has no way to know if the lock has been released or if the user is still modifying the file.
It is recommended to come out with your own policy regarding changing of files. e.g. each user can change the filename before modifying, and when finished, change the name back.
3. Is it a way to backup automatically the document whenever someone changed the document? thanks a lot!
I assume you mean someone changed files on DriveHQ server. In this case, no. But if files are on your local hard drive, yes.
To achieve backup on server and keep old versions; I suggest you use copy and paste to keep old version files.