We'll also need more information. Have you been sued? If you're not aware of any lawsuit, you should check your local court records carefully. It's possible the complaint itself was "lost" (served by leaving at residence and blew away in the wind, sent to wrong address, etc) but you've received this update. This would not be a rare occurrence.
If you haven't been sued, sending you some trumped up notification of entering appearance for additional counsel would be a very sketchy move (a threat, basically).
If you have been sued, prepare your defense. Usually, though, this is more an issue of "replacement" than "addition".