If you have extended a web application to an extranet or internet zone, and you add an external user to a group while logged in to the internal zone, then elect to send the standard welcome email (see below), the resulting email will send links to the internal zone url, not the external zone that the users will need.
This is a BUG, or at best a VERY poorly designed feature...
The problem exists in Microsoft.SharePoint.ApplicationPages.AclInv.SendEmailInvitation()*
One of the lines read:
string urlToEncode = base.Web.Url + "/_layouts/people.aspx?MembershipGroupId=" + SPHttpUtility.UrlKeyValueEncode(byID.ID.ToString());
The problem, of course is that base.Web will be the web that the current user is in, not the web that the extranet user will have rights to.
Other than replacing/editing the AclInv.aspx page in Layouts I don't see any way this can be fixed. Given that this thread has been out here for more than two years I hold little hope that MS will fix the issue any time soon...
(*locate the dll in C:\inetpub\wwwroot\wss\Virtual Directories\<your site>\_app_bin and reflect away...)