Sincerely, she'll be opposed to the idea of having humans go with them inside enemy territory. Even other servants dragging around behind her are a detriment to Atalanta's skill at operating independently unless they can keep up pace with her (which is nigh impossible, especially in a forest as impossible to cross as this one seems to be).

The idea of having humans follow the Servants into a death trap right at the heart of enemy territory is nothing short than asking for suicide.

Also, while I'm sure that there are people who can make it hard even for Atalanta to do her job, especially when it comes to Casters. You should also keep in mind that (among the canon Servants) she's the second best Hunter (losing only to Herc) and fights toe to toe with Arash for the title of best Bow(wo)man over all when it comes to actual Archery skills.

If there's something out there that even she would have difficult hunting while inside her favorite niche terrain, sending normal humans there would be nothing short of suicide which doesn't make any sense at all for me.