The only way to do this using built-in functionality is through Libraries in the Management module, and only if you are using the Premiere/Full licenses product. The sharing is done by creating Libraries and allowing other projects to Import them (through Management > Libraries). If you are using Enterprise or Starter editions the Library functionality is available, but the Import feature is not.
The Library functionality is described in the User Guide.
For ALM v11 refer to the User Guide, Chapter 11 Libraries and Baselines. There is a subsection titled "How to Use Libraries and Baselines". In Chapter 12 Imported Libraries there is a subsection titled "How To Share Libraries". Those sections provide the steps you need.
Note though that sharing libraries is available only for the Full/Premiere version of the product, and only for v10 and newer. You haven't said what version or what edition you have, so I don't know if the functionality is available to you.