Change welcome email for adding user to SharePoint group

Hi,

I have a requirement to change the welcome email body for adding user to SharePoint group.

I have come across articles which say that this can be done by changing the value of 'ServerEmailFooter' in file 'core.en-US' at location '~14\Resources'. However I want to change the message details like given below which are placed above the content of 'ServerEmailFooter' and is not a part of any resource files.

Welcome to the 'TestGroup' SharePoint group. System Account (SHAREPOINT\system) has added you to the 'TestGroup' group for this SharePoint site.
As a member of this SharePoint group, you can:
* Participate in the SharePoint site at: somesiteurl
* View the group home page at: somesiteurl

Please let me of know of how this can be done.