Keeping a blog updated can be hard work if you're doing it on your own. If you can assemble a team of bloggers, not only will the blog have more content, but you'll benefit from diverse ideas and writing styles. How do you make it happen?
First, the blog should be created by one person under a main Google account. Then, go to the "Settings" tab and the "Basic" subtab. Under Permissions is an option to Add Authors. Add authors by entering their email addresses – they'll need to have Google accounts for this to work, but if they don't, the invite will let them create one.
Then you need to decide what level to assign to the other authors. If you make them administrators, they will have full control over the blog. They can change the options, adjust the template, and add or remove authors (in addition to making and editing posts). Authors who aren't admins can still create and edit their own posts.