Using People Picker control in your Webpart

using System;
using System.Runtime.InteropServices;
using System.Web.UI;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Serialization;

using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;
using Microsoft.SharePoint.WebPartPages;
using System.Web.UI.WebControls;

namespace PeoplePickerWebPart
{
  public class TeamMemberPicker : System.Web.UI.WebControls.WebParts.WebPart
  {
      protected override void CreateChildControls()
      {
          try
          {
              PeopleEditor PeoplePickerControl = new PeopleEditor();
              PeoplePickerControl.AllowEmpty = true;
              PeoplePickerControl.AllowTypeIn = true;
              PeoplePickerControl.BrowseButtonImageName = "browse";
              PeoplePickerControl.BrowseButtonToolTip = "Pick an TeamMember";
              PeoplePickerControl.CheckButtonImageName = "Validate user";
              PeoplePickerControl.ErrorMessage = "No TeamMember Found";

              this.Controls.Add(PeoplePickerControl);
              base.CreateChildControls();
          }
          catch (Exception ex)
          {
              Literal _ErrorMessageLiteral = new Literal();
              _ErrorMessageLiteral.Text = "Custom Error: " + ex.Message;

              this.Controls.Clear();
              this.Controls.Add(_ErrorMessageLiteral);
          }
      }
  }
}

Articles:
http://dotnet.org.za/zlatan/archive/2008/04/21/using-peopleeditor-control-with-web-parts-in-sharepoint-2007-wss-3-0.aspx