<drive>:\<custom_folder_path>\Bin folder to Global Assembly Cache (GAC). This can be performed by the installer or using the Visual  Studio Command Prompt.

Use the following command to add the Dlls(example Test.dll) to GAC:

gacutil /i "<drive>:\<custom_folder_path>Test.dll" 

There are few other ways by which we can add assembly to the GAC.

2.By using .Net frame work configuration tool.(If you go to control panel -> Administartive tools, you will find Microsoft Frame work configuration).

Few more ways are there..