What if a collaborator makes changes I don't want to keep?
Overleaf allows you to save and label different versions of your document, just as you could with writeLaTeX. Simply save a version before you send them the link, and you'll always be able to restore it back to that point.
These saved versions can be seen & restored by any collaborators, but importantly, only you can delete them. This lets you share a doc without fear you'll lose work if it is mis-edited by anyone - if you save a version prior to sharing the doc, you've created a fixed restore point. An example use case is students who can save their work before sharing with their peers for review/help.
You can also share a read-only link to your document - this will enable collaborators and colleagues to review your work without being able to make changes.