How to Have Multiple Writers on Tumblrby Nick Peers
Only secondary Tumblr blogs can have multiple authors; there is currently no way to add members to a primary blog. You can create as many as 10 secondary blogs per day and add as many members as you want. Authors can post on your blog, as well as edit and delete their own posts. However, secondary blogs can't like posts, ask questions or be members of other blogs. Only you and your blog admins can invite and remove members.
Click your blog in the upper right corner of the dashboard and then choose "Create a new blog" from the menu.
Type a title for your new blog into the "Title" field and a URL into the "URL" field. Enable the "Password protect this blog" option if you wish to use it and then type a password. Click the "Create Blog" button.
Click "Members" below the name of your new secondary blog to navigate to the blog's Members page.
Type the email address of the first writer you wish to invite and then click the "Invite to this blog" button. The user becomes a member of your blog as soon as he accepts the invitation.