Did you ever find an answer?
If you compiled php5 using the instructions here: http://wiki.dreamhost.com/index.php/Installing_PHP5 then you compiled without pear. I'm not sure why they enable --without-pear in that script.
If you recompile but remove the --without-pear directive from the script, then you can use pear. However, you need to fix a few paths in the php.ini for the site using the custom php5.
Add or modify the following line:
include_path = ".:/home/username/php5/lib/php"
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Then you can install pear modules into the custom php5 using:
~/php5/bin/pear install -o --alldeps PEAR Log
(I think eGroupWare only requires the PEAR and Log modules.)
I am working on a HowTo for this and will add it to the wiki when I've got the kinks out.