How to convert privateassembly to Shared Assembly (Dot Net)

This famous concept is known as - "Global Assembly Cache".

Have a look at below links for more information on this.
http://msdn.microsoft.com/en-us/library/yf1d93sz%28v=VS.100%29.aspx

http://msdn.microsoft.com/en-us/library/6axd4fx6.aspx

To make assembly shared you need to strongly sign it and add it to GAC.

Select the check box from
project properties>>Signing tab>>Sign the assembly

Create a strong key
Choose a strong name key file: >> new
enter the Key file name:
enter password
reenter password
Strong name key will be created.

Alternate way to create strong name

sn -k "C:\[DirectoryToPlaceKey]\[KeyName].key" 

Host the signed assembly in Global Assembly Cache

gacutil -I "C:\[PathToBinDirectoryInVSProject]\myGAC.dll"