I just presumed Rangers wouldn't follow you to Big Mt. considering that they wont follow you inside certain locations in the Mojave but when I traveled to Big Mt. with a Ranger in my party she came with me. Has this happened to anyone else?

Haha that's awesome. Never happened to me but I might try it.

No that's probably not supposed to happen. Oh well, might as well enjoy your unintentional companion. Try not to get him/her killed.