Redmine offers multiple ways to manage clients. Either create a special role called client and expand it, or use some plugins, like the one described in this recipe. Considering this, if you are following Redmine's main use-case scenario of managing multiple projects simultaneously, it is good practice to install clients plugin, and be able to quickly find client's contact details.
To start using this plugin, follow these steps:
git clone https://github.com/splbio/customer_plugin.git plugins/customer_plugin
Do not confuse this fork of the plugin with the original Eric Davis-es repository which is ...