The easiest way to do this is to share the URL with your collaborators but change the end of the URL after the last slash to the word copy. Here is an example using a fake URL:

 

Original URL:
https://docs.google.com/document/d/abcdefg/edit

 

Delete everything after the last slash:
https://docs.google.com/document/d/abcdefg/

 

Type the word copy after the slash:
https://docs.google.com/document/d/abcdefg/copy

 

Now when you share the URL your collaborators will be asked to make a copy of the document (that's the only way they'll be able to access it). Each person will then have their own document -- it won't be shared with the other collaborators or with you. When they are ready to share their edits with you, they will have to share the document with you (because unless they share it with you, you won't have access to it).