I'm trying to create a multi user blog, where multiple authors can edit the same entry. Is this possible with Craft CMS? Is there a way to assign multiple authors to an entry? I only managed to assign one user as an author.
No, there can only be one author. But what you can do is to add one or more custom user fields to your entry type and use these to assign different type of users to your entry (all contributing authors, photographers, etc.).
Set user privileges like Douglas describes in his answer, to allow multiple authors to edit entries in your section.