If someone else owns a site and invites you to be an Editor, you will receive an email invitation and be able to access the site within the Recent sites area. You may want to select the Not owned by me option to find the site more quickly.

If you are editing someone else's site or participating in a group or class project, realize that you can see and edit all content, and other editors can see and edit your content. Only edit your own content or content that you have been asked to review and edit. DO NOT DELETE anyone else's work and do not publish the contents of the site unless you are the owner. The owner of the site will review and publish changes. If you accidentally make a mistake or remove items, please notify the owner of the site immediately.
Also, it's always a good idea to save a copy of your content (text and images) in another document so that if anything happens, you can recreate a page quickly.