At the time of this answer it does not appear to be possible to to hook into the Git author and email to parse out the email address with CCNET.
The workaround is to get a list of all the committers to your Git repository and enter them like so under the element. The name must match the name of the user's name in the commit exactly, including case. Do not specify a group for these users if you want them to receive emails only for builds in which they contributed a commit.
<email includeDetails="TRUE" subjectPrefix="CCNET Project ">
<from>mybuild@starfleet.com</from>
<mailhost>email.starfleet.com</mailhost>
<users>
<!-- Do NOT specify a group to send an email to these folks
if and only if they have committed changes that are part
of the current build. The 'name' much match the 'name'
used in the commit log exactly (case sensitive) -->
<user name="James Kirk" address="jkirk@starfleet.com" />
<user name="Bones" address="lmccoy@starfleet.com" />
</users>