Tuck is able to speak to Jack from anywhere in Jack's body - that's up to a good four or five feet from the receiver in Jack's ear. So how come Tuck can't pick up from or send to the receiver when he's in Lydia, but still within four feet?
For that matter, why was Tuck in Jack's mouth for the kiss?
Why did Jack's doctor, co-worker and boss get invited to Tuck and Lydia's wedding?
Probably because in the interim, they became Tuck and Lydia's friends too?
They needed more guests to fill out Tuck's side? Does Tuck strike you as the sort of guy that keeps close to his extended family.
The Doylist answer, of course, is that he could finally complete his transformation into a proper badass in his own right by telling them all to go screw themselves in awesome fashion. As for a Watsonian explanation, the co-worker at least got caught up in the chaos surrounding their adventure at least once during the movie (the nightclub scene), so they might have extended her an invitation as a way of saying thank you for the help / sorry for the inconvenience. Alternatively, Tuck and Lydia might have allowed Jack to bring along a couple of plus-ones so that he'd have at least a few people he knew at the wedding and wouldn't be entirely surrounded by strangers, and since his social life pre-movie doesn't exactly have seemed to be on fire these might have been the few people he knew well enough to invite.
So Tuck's vessel caught champagne that Jack drank, and Tuck didn't find it at all nasty or unhygienic to drink it himself? It contained Jack's saliva and bacteria!
Tuck's an alcoholic in a desperate situation. I don't think he'll have too much of a problem with that.