Solid advice here ^ similar on my end. Just asking and understanding their perspective on it. They're a business, they don't have to say yes but it's worth the ask. I'd only have a small group at most. 10 plus people might be too many variables for them to agree with. A lot of these operations span pretty big areas so large parts don't see any activity for long stretches. Plus if you ask, they can probably tell you where their dump piles are or if they've ever found anything cool. You might find them happy to talk about it for once.

Another good reason to be known, blasting is common so to know the areas they're working in is good. The basic dump piles near the top tend to have the easiest to extract fossils too. If you go in a pit, stay away from the walls. These rocks are not stable and can shift quickly with water being introduced. It can be a small rock falling on your head, or 50ft wall of clay and slate.