The first step would be to find the block(s) of code that are run when a member is promoted, either via automatic promotion because they've hit x number of posts, or manually by an admin.
Once you've found that, look for the closest hook, and write the plugin that will send a PM. If memory serves, there's a tutorial in the 3.5 Mod Tutorials section by Kirby or Marco (I forget which) that shows you how to send a PM.
Thanks,
ALan.
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