How to ask current grad student in a lab for advice about getting a position in the lab?
You can email a grad student but you may not get a reply, depending on lots of things, including how much effort it would take to answer.
You can ask about what the work is like. You can ask for the expectations of the PI. You can ask whether the work complements their dissertation work. You can ask about what skills are most useful in the lab, but this might be a hard question to answer.
Don't make it personal. In particular, I'd suggest avoiding questions about the PI's personality, and such. Don't ask for assistance in getting accepted into the lab or the degree program as it isn't their job to assist anyone else in that.