Quoting MPEG LA from streamingmedia.com:
Thus far, 12 parties have been found to have patents essential to the VP8 standard. Generally, parties may submit patents for evaluation of essentiality to VP8 at any time during the process of facilitating a patent pool, and may continue to do so after the pool launches, assuming one is formed.
Once a pool license is created, it typically remains open for the inclusion of additional essential patents, and patent holders with essential patents are welcome to join the license as Licensors. MPEG LA met with VP8 essential patent holders in late June to facilitate a discussion among them whether and on what terms they may want to create a VP8 patent pool license and is continuing to facilitate that discussion.