A new service called TweepML just launched that makes it easy to create and share groups of Twitter users. It's more than just a grouping service though, TweepML is also an open extensible format. What OPML is to RSS, TweepML is to Twitter. [...] In addition to the groups service, TweepML is also announcing an Open Standard Extensible format. This is an XML-based format, similar to the RSS grouping format OPML, for applications and services to share lists of Twitter users. An example use is publishers adding a "tweepml.tml" file to their site listing its writers. We will look to do that for ReadWriteWeb, using the above file.