Non-default assignments not on correct email list

Some of the elders in our ward attend high priests group. We have changed their assignment from EQ to HP. However, when looking at the list of high priests under directory, they do not appear. Therefore, when I try to email the entire HP group regarding a lesson or activity, they have to be manually added (and are sometimes missed.) How can I make sure that everyone attending HP group gets the email, regardless of their priesthood office?

I just checked and the Directory "Email All" feature seems to be the only place that does not have non HP in the mailing list.
If you have access to LCR or LDS Tools, all those who attend HP
meetings are listed, including Elders. You can use one of those to email your group.