I would suggest that perhaps "We are IN your wedding" should not be accepted as an answer, since (and maybe this is just my personal understanding) being AT a wedding and being IN a wedding are to very different things.
To be clear, being AT a wedding would mean attending as part of the audience. Being IN a wedding would mean being the bride, groom, a member of the wedding party, someone who takes an active role in the ceremony itself.
I don't know if this is a cultural thing but I thought it was a point worth bringing up at least.
In English, being IN a wedding means you are one of the official participants, as in: bride, groom, bridesmaid, usher (groomsman), minister, reader, etc. In other words, anyone with a specific role at the wedding. If you are just an observer (an invited friend), then you are AT the wedding.
Forgive my stupidity but I still don't understand how the context of talking to an individual makes it clear.
If I alone am talking to one person and I say it, then it's clear that I mean us two. But let's say that a friend and I are talking to a single other person. Context doesn't really make it clear. Is it the wedding of the person we are talking to, or someone else?
You're right, I wasn't very clear. Talking to one person, it would be "bryllupet ditt" if you meant the person you were talking to, so if you used "deres" it could only mean "theirs" and not "yours(plural)". If you are talking to TWO people, "deres" could be either "yours" or "theirs" but hopefully from the context of the conversation, it would be clear. So, in the case of this exact sentence, if you were talking to one person, "deres" could only mean "their" but if you were talking to two (or more) people, it could mean either without clarifying context.
So after reading all the comments here, I am still not clear which meaning "i bryllupet" has. It is "AT the wedding", or "IN the wedding"? The Norwegian word "i" generally means "in", but in this case, perhaps it really means being AT the wedding, not IN the wedding (as people have noted, in English, those are two very different things). And if "i bryllupet" here really means "AT the wedding", is there a Norwegian way to say "IN the wedding", in the English sense of meaning "having a specific role in the wedding ceremony"?