On occasion, you might want to invite a guest blogger to participate in your blog. There are two approaches to enable this scenario.
The first option is to simply add the person as an author of the blog temporarily. In this case, the user has permissions to access all posts within the given blog.
If the guest blogger is syndicating their content from another blog, you could ask that they setup an RSS feed to represent content that should be mirrored to your blog and setup that feed as a mirrored feed. See How can I mirror content from another blog or RSS feed?