From version 1.2.4 on, Horde Groupware and Horde Groupware Webmail Edition ship with a complete, working PEAR installation. This means that you can update the PEAR packages bundled with Horde Groupware, for example to get bug or security fixes directly from PEAR. You can also install new PEAR package that might be necessary for any additional Horde applications that you plan to install.
To get the local PEAR installation working, you have to run the setup.php script and pick the menu item "Update PEAR for a new or changed location". This is also necessary each time you rename or move the directory of Horde Groupware. Then you can manage the bundled PEAR installation by calling the PEAR command like so:
./pear/pear -c pear/horde.ini install Text_Wiki
This example assumes that you are inside the Horde Groupware directory, but you can call the pear command from anywhere. Just make sure that you use the pear command from inside the Groupware's pear/ directory, and the -c flag with the horde.ini file from the same directory.
See http://pear.php.net/manual/en/guide.users.commandline.cli.php for further documentation of the pear command.