There is this lab that I want to work for as a graduate research assistant (I'm a masters and hoping for research experience and funding) and before I email the PI, I thought of emailing the grad student who was already working there to get a better understanding of the lab and what kind of skillset that I should develop to make myself more competitive. With that being said, how can I email the grad student without coming off as annoying or ingenuine?
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.