Lately, I've started hanging out with a group of people who are a lot more knowledgeable than I am about most things Jewish, and while I really like them, I'm embarrassed to ask them what's going on and end up being left out of a lot of the conversations. How can I ask without looking dumb, or how can I find out what they were talking about after the fact?
It's like learning a new language
Sometimes, people use "insider language" to show that they belong and to make that fact clear to anyone who's listening. Sometimes, though, people use insider language because they're so far inside whatever community, or group of friends or whatever that they don't even realize that they're doing it. I have certainly encountered both types in various Jewish circles. It's unclear from your letter which type you're dealing with, but since you're choosing to be friends with them, I'm going to assume they're not trying to make you feel like an outsider, and I'll proceed as if you're talking about the latter category.
If your ego can take it, your best option is politely and consistently to say, "Can you translate that?" or "I'm not sure what that is." If they're genuinely kind people, they'll learn to anticipate your questions and will naturally start to translate as they go. You don't need to apologize for not being familiar with the same cafe in Jerusalem that everyone else is talking about or for not knowing what kitniyot means and why people talk about them all the time. (I guarantee I'm not familiar with the cafe either, and kitniyot are the category of legume-like foods that Ashkenazi Jews don't eat on Passover. I know Passover's over, and I know you didn't ask me about kitniyot, but it's incredible how I often I answer that question, so I figured it might prove useful to you at some point.)
Are there one or two individuals in this group with whom you've formed an especially close friendship? Maybe you could talk to someone one on one and first ask for all the translations and explanations you've been wondering about and then ask how to get better at being part of the group's conversations. The general opinion might be that you're just quiet, or, worse, that you're not interested in what they have to say, so it may be helpful to get a read on where you fit in and have to have an ally in place the next time you all get together.
What I strongly urge against you doing is to keep hanging out with them silently, hoping that eventually you'll understand everything that's going on, because unless you proactively ask your questions or spend a couple years in yeshiva, there may just be a knowledge gap there that you can't fill in casual conversation. I know, Passover is over, but the references just keep coming: One of my favorite aspects of the seder is the idea that we can celebrate freedom through asking questions. Jews value the process of asking, and it's possible that others in the group are wondering the same things as you and will be relieved that you had the guts to speak up.
I actually find it really empowering in these settings to admit to what I don't know, and I enjoy the conversations so much more once I understand the key concept that I've been missing. If your friends do anything to make you feel dumb, perhaps you need to reevaluate the friendships, but if the perception of judgement is in your head, dispense with it, ask your questions, and move on. Even the most knowledgeable Jews (and everyone else) always have something to learn, so consider yourself in good company.