Last 2014, Jetpack 3.3 introduced a centralized dashboard that enables users to manage Jetpack powered sites wherein you could only do the following:
- Plugin Management
- Initiate plugin updates
- Automatic updates
But now, you can browse and install plugins from the WordPress plugin directory on sites you own that use Jetpack with Manage module activated. Visit https://wordpress.com/plugins/browse and choose the site you want to manage. Check for the Configure area on the sidebar and click the Add button to open the plugin browser. A plugin browser to search for and install plugins is another feature that makes Jetpack Manage a free, decent alternative to commercial services for managing multiple sites.