Team blogs are quite useful when small groups of people wish to contribute to a single blog. Basically, one person initially creates the blog, then adds other people to it as members.
Team members can either be administrators or not. Administrators can edit all posts (not just their own), add and remove team members (and grant admin access), and modify the blog's settings and template. Non-admins can only create and edit their own posts.
Here's how to add members to your blog:
First, find the "Permissions" section under Settings | Basic.
Then click Add Authors:
Next, type the email addresses of the people you're inviting to the blog, separating each address with a comma. They'll receive an email with a confirmation link soon. Note that they must have Google Accounts, and if they don't already, they'll be prompted to create one.
When you're ready to send the invitations, click OK.