1. In your Google Form, click on the three dots on the upper-right of the screen. A menu will appear.
Click the three dots.
2. Choose "Add collaborators." A window will pop up. The window includes a sharing link, but you have to add someone as a collaborator before they can access the link. You can email invitations to individuals, just as you would with a Google Docs.
3. To make the form public, click on the word "Change…" at the bottom of the window. A new window will appear. The default setting for your form will be set to private, so that it can only be shared with specific people. However, you can change this so anyone with a link can collaborate. You can also make the form public on the web.
4. When you are done adding collaborators, click the blue "Save" button at the bottom-left of the window to close it.
Note that, unlike in a Google Docs, you can't limit the access of your collaborators to view only or comment only. Anyone you share a form with as a collaborator can edit the form and the responses.