I mean surely she was still in contact with them; after all, she was faithful herself, wasn't she? Also there was a lot of time between Ar-Pharazon's Great Fleet and the Downfall of Numenor: approximately 36 days I believe it was. Yet she chose not to join them on the ships and tried to reach the top of Meneltarma (which was on fire at the time of the downfall). So why didn't she meet up with Elendil and the rest of her kinsmen?
The most plausible explanation is that forces loyal to Sauron prevented her; we know that they tried to prevent Elendil from leaving:
Elendil had remained in Romenna, refusing the summons of the King when he set forth to war; and avoiding the soldiers of Sauron that came to seize him and drag him to the fires of the Temple, he went aboard his ship and stood off from the shore, waiting on the time.
The Silmarillion IV Akallabeth
Even if we suppose that Míriel was in contact with the Faithful, it would presumably have been difficult for her to leave the Palace to join them, especially since (given who her father was) she was almost certainly suspected of having sympathies with enemies of the state.
Finally, I'll point out that Tolkien's later writings cast doubt on the assumption that Míriel's loyalties were with the Faithful; some late scribbles introduce the idea that Míriel actually did love Ar-Pharazon, in which case she presumably stayed behind for personal reasons. Christopher Tolkien comments:
It is not perfectly clear to me how the textual puzzle presented by these writings is to be resolved, but I am inclined to think that, contrary to appearance, the texts (a), (b), and (c) in fact followed the writing of the long rider to the Akallabeth, and that they represent the emergence of a doubt in my father's mind whether the marriage of Pharazon and Zimrahil was indeed 'against her will', and the sketching of a new story on the subject.
History of Middle-earth XII The Peoples of Middle-earth Part One Chapter 5: "The History of the Akallabeth"