What is the Blogosphere?
Internet Marketing Dictionary | Blogging and the Blogosphere

The blogosphere is a collective name for all blogs on the internet. In other words every single blog is part of the blogosphere.

In general use the word blogosphere refers to the moods, opinions and reactions of popular bloggers and especially people blogging about a specific event or episode in current affairs such as a hot news story or political development.

Inevitably some bloggers and therefore some blogs carry more weight on the internet than others, just as some newspapers and journals are more presitigious than others. Within the blogosphere there are few barriers to entry and so, with the notable exceptions of blogs published on well established media sites like the BBC, the importance of a blog and its significance within the blogosphere will depend on the quality of its content and the authority, wisdom, style and popularity of its author.

To join the blogosphere all you need to do is start blogging.