Also, just to be clear, Dmitri is blatantly ignoring Solomon, and by extension Trinity, due to past occurrences of a Solomon deciding a job wasn't worth the risk to their charge, and convinced their hunter to back out of a job, leaving on his back heal before the show even started. Trinity isn't quite assumed to be of the same stock, but he's focused on moving out of the area that was recently assaulted, so introductions will get to wait. 

Also, he's specifically [u]not[/u] inviting anyone but Anya to show up, but if you want come anyway, he won't turn down the help. The Devil he's targeting is small and relatively weak, but it's fast, agile, and can become invisible at will. Not something for a human to take on lightly, since one mistake, is literally game over for you. A vampire on the other hand, takes a bit more to kill, so Anya has much better odds of coming out the other side on this one.