title: How to Custom Group By multiple records according some criteria
body: <p>please consider this scenario:</p>
  
  <p>I have data like this :</p>
  
  <pre><code> Id          Group            Value
   -----------------------------------
   1            1                100
   2            1                120
   3            1                100
   4            2                90
   5            2                105
   6            3                300
   7            4                123
   8            4                110
   9            5                100
   10           5                110
  </code></pre>
  
  <p>I want to do <code>GroupBy</code> on this data as <code>1,2,3</code> place in one group and <code>4,5</code> place in another group. How I can do this with <code>LINQ</code>?</p>
  
  <p>thanks</p>
  
  <hr>
  
  <p><strong>Edit 1)</strong></p>
  
  <p>I want to place <code>1,2,3</code> in one group and <code>4,5</code> in another group and perform <code>Count</code> aggregation on them and I get <strong>6</strong> for first group and <strong>4</strong> for secnd group</p>