There is a Microsoft article that explains the background to the issue:

http://blogs.msdn.com/b/vsto/archive/2010/03/08/deploying-your-vsto-add-ins-to-all-users-saurabh-bhatia.aspx

However to simply have our MS-Office Add-In be available for other users, simply add the following keys to their user's hive in the Windows registry:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\SpiraExcelAddIn]
@="SpiraTeam Importer for MS-Excel"

[HKEY_CLASSES_ROOT\SpiraExcelAddIn\CLSID]
@="{42E57061-6005-4F7F-BAAC-D47CB07A49A1}"

[HKEY_CLASSES_ROOT\CLSID\{42E57061-6005-4F7F-BAAC-D47CB07A49A1}]
@="SpiraTeam Importer for MS-Excel"

[HKEY_CLASSES_ROOT\CLSID\{42E57061-6005-4F7F-BAAC-D47CB07A49A1}\InprocServer32]
@="C:\\Program Files\\Common Files\\Microsoft Shared\\VSTO\\10.0\\VSTOLoader.dll"
"ThreadingModel"="Both"
"ManifestLocation"="C:\\Program Files\\Inflectra\\Spira MS Office Add-Ins\\"
"ManifestName"="SpiraExcelAddIn.dll.manifest"

[HKEY_CLASSES_ROOT\CLSID\{42E57061-6005-4F7F-BAAC-D47CB07A49A1}\ProgID]
@="SpiraExcelAddIn"

[HKEY_CLASSES_ROOT\CLSID\{42E57061-6005-4F7F-BAAC-D47CB07A49A1}\Programmable]

[HKEY_CLASSES_ROOT\CLSID\{42E57061-6005-4F7F-BAAC-D47CB07A49A1}\VersionIndependentProgID]
@="SpiraExcelAddIn"

[HKEY_CURRENT_USER\Software\Microsoft\Office\Excel\Addins\SpiraExcelAddIn]
"CommandLineSafe"=dword:00000001
"Description"="SpiraTeam Importer for MS-Excel"
"FriendlyName"="SpiraExcelAddIn"
"LoadBehavior"=dword:00000003
"Manifest"="C:\\Program Files\\Inflectra\\Spira MS Office Add-Ins\\SpiraExcelAddIn.vsto|vstolocal"

[HKEY_CLASSES_ROOT\SpiraWordAddIn]
@="SpiraTeam Importer for MS-Word"

[HKEY_CLASSES_ROOT\SpiraWordAddIn\CLSID]
@="{D1D4C344-846F-4B8F-97D0-074B58B8915B}"

[HKEY_CLASSES_ROOT\CLSID\{D1D4C344-846F-4B8F-97D0-074B58B8915B}]
@="SpiraTeam Importer for MS-Word"

[HKEY_CLASSES_ROOT\CLSID\{D1D4C344-846F-4B8F-97D0-074B58B8915B}\InprocServer32]
@="C:\\Program Files\\Common Files\\Microsoft Shared\\VSTO\\10.0\\VSTOLoader.dll"
"ThreadingModel"="Both"
"ManifestLocation"="C:\\Program Files\\Inflectra\\Spira MS Office Add-Ins\\"
"ManifestName"="SpiraWordAddIn.vsto|vstolocal"

[HKEY_CLASSES_ROOT\CLSID\{D1D4C344-846F-4B8F-97D0-074B58B8915B}\ProgID]
@="SpiraWordAddIn"

[HKEY_CLASSES_ROOT\CLSID\{D1D4C344-846F-4B8F-97D0-074B58B8915B}\Programmable]

[HKEY_CLASSES_ROOT\CLSID\{D1D4C344-846F-4B8F-97D0-074B58B8915B}\VersionIndependentProgID]
@="SpiraWordAddIn"

[HKEY_CURRENT_USER\Software\Microsoft\Office\Word\Addins\SpiraWordAddIn]
"CommandLineSafe"=dword:00000001
"Description"="SpiraTeam Importer for MS-Word"
"FriendlyName"="SpiraWordAddIn"
"LoadBehavior"=dword:00000003
"Manifest"="C:\\Program Files\\Inflectra\\Spira MS Office Add-Ins\\SpiraWordAddIn.dll.manifest"

[HKEY_CLASSES_ROOT\SpiraProjectAddIn]
@="SpiraTeam Importer for MS-Project"

[HKEY_CLASSES_ROOT\SpiraProjectAddIn\CLSID]
@="{40D74C58-3298-4EA2-BE82-E1B78A9A549C}"

[HKEY_CLASSES_ROOT\CLSID\{40D74C58-3298-4EA2-BE82-E1B78A9A549C}]
@="SpiraTeam Importer for MS-Project"

[HKEY_CLASSES_ROOT\CLSID\{40D74C58-3298-4EA2-BE82-E1B78A9A549C}\InprocServer32]
@="C:\\Program Files\\Common Files\\Microsoft Shared\\VSTO\\10.0\\VSTOLoader.dll"
"ThreadingModel"="Both"
"ManifestLocation"="C:\\Program Files\\Inflectra\\Spira MS Office Add-Ins\\"
"ManifestName"="SpiraProjectAddIn.vsto|vstolocal"

[HKEY_CLASSES_ROOT\CLSID\{40D74C58-3298-4EA2-BE82-E1B78A9A549C}\ProgID]
@="SpiraProjectAddIn"

[HKEY_CLASSES_ROOT\CLSID\{40D74C58-3298-4EA2-BE82-E1B78A9A549C}\Programmable]

[HKEY_CLASSES_ROOT\CLSID\{40D74C58-3298-4EA2-BE82-E1B78A9A549C}\VersionIndependentProgID]
@="SpiraProjectAddIn"

[HKEY_CURRENT_USER\Software\Microsoft\Office\MS Project\Addins\SpiraProjectAddIn]
"CommandLineSafe"=dword:00000001
"Description"="SpiraTeam Importer for MS-Project"
"FriendlyName"="SpiraProjectAddIn"
"LoadBehavior"=dword:00000003
"Manifest"="C:\\Program Files\\Inflectra\\Spira MS Office Add-Ins\\SpiraProjectAddIn.dll.manifest"