It probably depends a lot on your personal style.
If it were me I'd probably go the route of pursuing a friendship first - while making it completely obvious that I have a boyfriend and this friendship is not a secret - then flirting like mad. I wouldn't necessarily get into a deep discussion about relationships unless the conversation turned that way and wouldn't use the word polyamory at all unless until it was obvious that things were headed in the "relationship" direction (I'd use "non-exclusive" if I had to put it to words).
I hung out with lots of guys in college when I had a boyfriend (I generally find it easier to make friends with men) although I wasn't having sex with other guys at that point. Generally, I'd ask them if they wanted to join me for lunch or coffee or whatever, then I'd make a quick call to MrS and say "Hey, I won't be coming home until later, I'm having coffee with MrInteresting from English class that I was telling you about" - MrS would say "Cool, we all are going to be hanging out at the pool hall until 6 or so if you guys want to stop by." This let the guys know that there was a boyfriend in the picture and that it wasn't an issue without saying it in so many words.
PS. My reply post got a bit long and off-topic so I posted the rest of it in my "Notebook Blog" here.