Just looking for some advice for someone new to polyamorous relationships.

I recently found myself hardcore flirting with a friend who I know has a girlfriend, and when I checked to see if I was crossing lines and we were actually flirting, they offered me a spot in their bed. With both of them.

I’m not opposed to the idea - still a lil in shock, honestly. I’m looking for advice - what are some thing you wished you knew going into a polyamorous relationship? Things to watch out for? Lines to make sure aren’t crossed? Any and all advice is appreciated!