using (PrincipalContext ctx = new PrincipalContext(ContextType.Domain))
{
   // define a "query-by-example" principal - here, we search for a GroupPrincipal 
   // and with the name like some pattern
   GroupPrincipal qbeGroup = new GroupPrincipal(ctx);
   qbeGroup.Name = "GroupPrefix*";

   // create your principal searcher passing in the QBE principal    
   PrincipalSearcher srch = new PrincipalSearcher(qbeGroup);

   // find all matches
   foreach(var found in srch.FindAll())
   {
       // do whatever here - "found" is of type "Principal"
   }
}