Dr. Moira MacTaggert was part of, but not key to, both X-Men: The Last Stand and X-Men: First Class, so it would stand to reason that a movie that serves as a de facto sequel to both could easily incorporate her, but that she wouldn't strictly speaking be necessary.
The universe-building that Mark Millar has suggested he and Fox want to do with these films is probably well-served by keeping some of the people on multi-picture deals out of the big team movies so that they're on-hand for the next one, or the next Wolverine solo outing, or whatever comes next.
"I haven't heard yet what's happening--I'm waiting to hear," Byrne told Hey U Guys during a red carpet interview in promotion of her new romantic comedy I Give It A Year.
The interviewer, at the time, seemed to have assumed that Byrne was in for the sequel, but given the time-jumping nature of the film, and Jones saying that she hasn't been asked back yet, it begins to look more likely every day that some members of each "generation" of X-Men will be trimmed from the cast in order to make the movie a bit more manageable.
You can check out the interview below. X-Men: Days of Future Past is due in theaters July 18, 2014.