AddUser method doesn't contain send email functionality.we can use sputility.sendemail to configure the mail.the following code will help.
string groupval = sitePath + "/_layouts/people.aspx?MembershipGroupId=" + SPHttpUtility.UrlKeyValueEncode(group.ID);
if (group != null)
StringDictionary headers = new StringDictionary();
headers.Add("subject", "Welcome to the SharePoint group: " + group + " for site: "+web.Title+"");
headers.Add("content-type", "text/html"); //This is the default type, so isn't neccessary.
string bodyText = "Welcome to the '" + groupName + "' SharePoint group. " +group.Owner +
"has added you to the '"+groupName+"' group for this SharePoint site.<br/><br/>As a member of this SharePoint group, you can:<br/>* Participate in the SharePoint site at:<a href=" + site.RootWeb.Url + ">" + site.RootWeb.Url + "</a>" +
"<br/>* View the group home page at:<a href=" + groupval + ">" + groupval + "</a>"+"<br/><br/>What is a SharePoint site?"+"<br/>A SharePoint site is a Web site that provides a central storage and collaboration space for documents, information, and ideas. A SharePoint site is a tool for collaboration, just like a telephone is a tool for communication, or a meeting is a tool for decision making. A SharePoint site helps groups of people (whether work teams or social groups) share information and work together. For example, a SharePoint site can help you: <br/>"+
"<br/>* Coordinate projects, calendars, and schedules.<br/>* Discuss ideas and review documents or proposals.<br/>* Share information and keep in touch with other people.<br/><br/>SharePoint sites are dynamic and interactive -- members of the site can contribute their own ideas and content as well as comment on or contribute to other people's. ";
SPUtility.SendEmail(web, headers, bodyText);
This code will give the exact mail as out of the box