His friends keep bringing up inside jokes you don't get: what to do

When he mentions an inside joke without explaining it, the first thing to work out is whether he's doing it on purpose or just talking the way he always talks with these people. Usually it's the second. He's writing to his friends, or retelling something from a group chat, and the context is so familiar to him that he doesn't translate it. His message is not really addressed to you.

The signal to watch is what he does next. If he drops a reference and then, a message later, gives you a sentence of context, he's already thinking about you as someone who needs to be brought along. If he keeps going with the other person or the group without ever looping you in, that's worth paying attention to over several conversations, not one. One unexplained joke means nothing. A pattern where you are repeatedly the one who's out of the loop, and he never seems to register it, tells you something about how much he's thinking about your side of the conversation.

A good reply here follows the thread just far enough to show you're interested. You don't need to know the whole Lisbon story. You need to show that you noticed, that you're amused or curious, and that you're happy to hear it at whatever length he wants to give it. That is different from asking him to explain something he clearly doesn't want to unpack by text.

The other job is making it easy for him to say more. People tend to tell stories when someone signals they'd like to hear one, and a single short question is enough. What you want to avoid is a string of questions that turns a passing joke into homework. If he gives you a little, take it and move on. If he gives you nothing, that's fine too. You can always ask in person, where a story usually lands better anyway.

Texts about old group stories often go nowhere because the story needs a face, a laugh, and some back-and-forth to land. If he's the kind of person who gets animated telling a tale, the best outcome is usually an in-person conversation where he can act it out and you can react. If you push for the whole thing over text, you tend to get a flat paragraph that makes the joke sound smaller than it is.

So if the reference keeps coming up and you still don't know what it means, say something light like wanting to hear the full version when you see him. That keeps the door open and gives him a reason to plan time with you. It also shows you're not trying to decode his friendships through his phone.