True, but Dorin's family settled in the Blue Mountains following the whole arrival of Smaug thing. Doubtless he and others there would've heralded a summons from Thorin, but I'd have thought the Battle would have long concluded by the time they could get there.
Well, I imagine Dorin getting to know Vigdis over the course of his visits to Dale is a possibility, and he could've run into Bersha and Inglord on the road. But as to how likely it is that the other characters could've encountered each other in the past, I imagine their players likely know them characters better than I do.
If A Ayelech and CommanderNecro are good with this idea we might have the basis of the company coming together. For the festival I imagine accomodations are scarce so perhaps Vigdis would offer to put Dorin up, and his guests as well and then it would be seeking out Calanhir at the festival.