In my current project I wanted to create a webpart where I could add users to a certain group in a site from a single adminstration site. Furthermore I wanted to use the nice and elegant People Picker. This control is called “PeopleEditor” which belongs to the Microsoft.SharePoint.WebControls namespace. This is how it looks like when ran in a webpart :

Here is the code to get the values from the control :

PeopleEditor _peoplePicker = new PeopleEditor();
_peoplePicker.AllowTypeIn = true;
_peoplePicker.AllowEmpty = false;
_peoplePicker.MultiSelect = false;

SPWeb web = new SPSite("urlToWeb").OpenWeb();
SPGroup group = web.SiteGroups["groupname"];

web.AllowUnsafeUpdates = true;

if (Group.Users.Count > 0)
{
      SPUserCollection users = group.Users;
      foreach (SPUser user in users)
      {
           group.Users.Remove(user.LoginName);
           group.Update();
      }
}
   
            
try
{
      group.AddUser(_peoplePicker.Accounts[0].ToString(), "", "", "");
      group.Update();
}

catch (ArgumentException error)
{
      //Do ErrorHandling here
}

finally
{
     web.AllowUnsafeUpdates = false;
     web.Close();
     web.Dispose();
}

And here is the code to set the values from the control :

if (group.Users.Count > 0)
{                
      foreach (SPUser user in group.Users)
      {                   
             _peoplePicker.CommaSeparatedAccounts= user.LoginName;                
      }                
}

Took me a couple of hours to find out I had to use the CommaSeperatedAccount property. Since you are getting the values using the Accounts or the Entities properties, it seems a bit odd to use this method to set the values.