My very first marriage in Skyrim, ever, ever, ever, was to Sylgja (the miner from Shor's Stone). And I had the whole Shor's Stone crew show up, a Riften Guard, Alvor, Faendel, Camilla Valerius, Lydia, that wandering bard guy, Ondolemar (wtf), some mod companion I hadn't picked up as a follower yet (whose only task ever turned out to be attending my wedding, as they were broken and got deleted soon after), Anneke Crag-Jumper, and Beirand. Beirand, btw, is the Solitude blacksmith, and I had not even visited Solitude on that character. The church was absolutely PACKED.
Second character I got married on, I married Jordis. I was level 40-something at the time, halfway through the Imperial questline, pretty much solved the problem of everyone living in Solitude or the surrounding area, and ditto for Whiterun and Markarth. The only attendee was Sorex Vinius (who gave me 3 gold when I walked near him as well, saying that it didn't mean we were engaged).
Next marriage was Marcurio. Basically, most of the inhabitants of the Bee and Barb showed up, including that Argonian that hates you, and Loius Letrush, who was broken and attacked me in the tavern, and out in the town, but not at the wedding (he had SOME decency, I guess).
I thought that maybe it had something to do with your quests in addition to your spouse's 'connections' in the world, which would have been why Sylgja brought the Shor's Stone people and her mother (but not her father? I guess he hated the idea of his little girl tying the knot...), and why Marcurio brought the residents of the Riften Tavern, since I'm guessing they're basically reenacting episodes of Cheers in there whenever you aren't around. It would also explain why Jordis had nobody (poor Jordis), since she was a housecarl that isn't even in the game until you become Thane, but you'd have thought that would have at least brought some of the palace people.
Anyway, that made sense. But then I married Brelyna Maryon, and...Sorex Vinius showed up. None of the college people, even though I was friendly with all of them.
Married Farkas. Aela shows up, but no other Companions. Also, Sorex Vinius.
Shahvee. Some Argonians from the Assemblage, one of the racist beggars, the Dunmer that they're yelling at when you first come into Windhelm. And Maul.
Gilfre. Uthgerd, Ysolda, and Sorex fluffing Vinius. At this point, I'm starting to think that maybe I should be marrying Sorex, because he's the only person who troubles to attend my wedding regularly out of the millions of people I help (and in at least one of the playthroughs, I'm pretty sure I had never even done his quest).