A guy I like is polyamorous, but I don't know what that really means to him and his girlfriend

I have some questions because I’m interested in a guy who is in a polyamorous relationship . I am is monogamous but would be okay with being with him while he was still in his current relationship. Is it usual that a polyamorous couple would let someone in who is only interested in one of them? Or is it then called an open relationship? I don’t want to approach them if it’s offensive in any way to his girlfriend.

It is very common for people in a polyamorous relationship to date other people without their existing partners being involved in that dating relationship. This is called V or “Vee” polyamory. The distinction between “polyamory” and “open relationship” is fuzzy and differs between people and is therefore not super useful here; you’re talking here about “triad” vs. “V” polyamory.

However, just because it’s super common doesn’t mean that it’s the specific type of polyamory this man practices. Your best bet is to ask him: “Hey, I’m interested in you, and I know you and your partner identify as polyamorous - what does that mean for you? What would that mean for people you date? What are your terms and expectations for new partners?” You can ask this with a sort of general-case curiosity that gives you plausible deniability, but it will be easy for this conversation to shift into you expressing a more personal interest if it goes well.

The thing about polyamory is that it requires lots of communication. There’s no way I can tell you how this particular couple understands their polyamory; you’ll just have to take that leap and ask him! If he tells you that yes, his relationship allows for him to date people without them also being involved with his partner, great! Don’t think about it as him and his girlfriend “letting you in” - if you dated this man, you would not be a newcomer or guest in an already established relationship, you’d be forming a new relationship between you and him. If he tells you that actually, he and his girlfriend only date as a couple, you’ll have your answer, and you can respond with gracious disappointment. That wouldn’t be “offensive,” and if she takes issue with you for asking a clarifying question about his polyamory, that’s her problem - you would not be doing anything wrong in this situation.