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How exactly does the crossposting of the comments work?

There are two different cases: 1. A comment is posted and approved in the blog with the original post. In this case it is stored in the database to which blogs the post was crossposted. The Xpost plugin will then transmit the comment via XML-RPC to these blogs. 2. A comment is posted and approved in a blog as response to a crosspost. In this case the Xpost plugin has stored the address of the blog containing the orginal post together with the crosspost. The comment will then be transmitted via XML-RPC to the original blog. The Xpost plugin in the original blog know to which other blogs the comment has to be posted and does so via XML-RPC.

I want to contribute. How can I do this?

The plugin is hosted on gitorious.org. You may create a clone of the repository, make your changes and request a merge afterwards.