Step 1 : Go to your blogger dashboard and select the blog on which you want to allow multiple authors.
Step 2: Go to Settings >> Basic. A page loads, Look for the "Permissions" section within the page. Here next to "Blog Authors" you would find your name and email listed as Admin.
An email will automatically be sent to all those persons you have invited. All they will need to do is to sign in with a Google Account to confirm the invitation and start posting to you blog.
Note: The persons you will be inviting will not be assigned administrator privileges (which will remain with you). They would neither be able to delete or modify any content which already hosted nor make any changes to your blog. The only thing they are allowed to do is to post content to your blog.