Overleaf makes sharing easy. On Overleaf v1, all you have to do is send them the link, and they can see and edit with you - instant collaboration!
The link to your document is the unique url created when you first started your project - simply copy the url from the web address bar, or use the share menu in the editor to find the link. You can also share a read-only link if you'd like to give others the chance to read your document without editing.
If you're worried your collaborator might mess up your work, simply save a version before you send them the read-and-edit link, and you'll always be able to restore it back to that point. Your projects are always private when you create them - no one can see your work unless you choose to share it.
Overleaf v2 allows sharing with links, or confidentially via email. There is some information on this here.