Add multiple users to People or Group field of a list using PowerShell

[Microsoft.SharePoint.SPFieldUserValueCollection]$lotsofpeople = New-Object Microsoft.SharePoint.SPFieldUserValueCollection            
$user1 = $w.EnsureUser("domain\user1");            
$user1Value = New-Object Microsoft.SharePoint.SPFieldUserValue($w, $user1.Id, $user1.LoginName)            
$user2 = $w.EnsureUser("domain\user2");            
$user2Value = New-Object Microsoft.SharePoint.SPFieldUserValue($w, $user2.Id, $user2.LoginName);            
$lotsofpeople.Add($user1Value);            
$lotsofpeople.Add($user2Value);            
$i["lotsofpeoplefield"] = $lotsofpeople;            
$i.Update();            
#-or-            
$l.Fields["lotsofpeoplefield"].ParseAndSetValue($i,$lotsofpeople);            
$i.Update();

Where $i is the list item to update. $w is the SPWeb object.

Reference - http://social.technet.microsoft.com/wiki/contents/articles/20831.sharepoint-a-complete-guide-to-getting-and-setting-fields-using-powershell.aspx#Set_and_Get_a_Multi-Person_Field