Because it is fun.
MyFaces and the RI (from Sun) are available at no cost. But people had to invest time and money to create and maintain these implementations. It just seems fair to give back something, doesn't it?
Sharing components is one way to earn "kharma" in the Apache-world. It also can be usefull as a reference while searching for a job.
Great. Just write an email about your contribution in the MyFaces user-mailing list. The feedback is usefull...
The next step is an email to one (or more) of the project-admins (see projects member page). This email must contain your sourceforge member name. The admins will then add you as project member.
Put on the user's hat (a fellow developer, so it should not be too difficult ;-) ). What would you like to have, when you want to reuse somebody else's stuff?
Either you come online in the irc-channel (support-page), or ask on the MyFaces mailing list, or write the admins an email.