The sample code as follows.

    SPWeb web = new SPSite("Site URL").OpenWeb();
    SPUserInfo[] userInfo = new SPUserInfo[1];
    userInfo[0].Email = "email@microsoft.com";
    userInfo[0].LoginName = @"domain\username";
    userInfo[0].Name = "username";
    userInfo[0].Notes = "Test";
    web.Roles["Reader"].Users.AddCollection(userInfo);