There's not really any formal way to join or contribute to XviD - write some code, post it to the devel mailing list, and if it works (well) it'll probably get committed in some form. Point optimizations (such as only rewriting or creating assembly for a single function, like an RGB->YV12 conversion) are the most common and easiest bits of code to create.
I don't know of any concrete plans to work on a H.264 codec, but if it's done I know that any coding experience is welcome.
-h
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