Could be anyone except Spike and maybe Smolder. They'd probably have invitations of their own. There's some pretty funny things you can do with that. Imagine her bringing her father along, or Garble, or that burping dragon, or that big, blue smiling one...

There are tons of different things you can do with these characters, most of which would contribute to the absolute mayhem that will ensue. Torch could be this ominous force sitting outside that freaks out the guests and still has personal beef with the princesses. Bonus points if Flash Magnus is present as well.

He's honestly the most obvious choice. He has a personal connection to Ember, personal reasons to be the source of conflict and they wouldn't have to animate him as much as other characters because he wouldn't fit inside.

Garble could be there against his will for friendship reasons and his attempts to ruin the evening either result in karmic slapstick or things actually going better. The smiling dragon could take to the different customs surprisingly well and be this well-mannered gentleman in the midst of the chaos.

In the mean time the other leaders would be causing problems of their own. Grampa Gruff would be complaining the whole time and say he had a nice evening in the end, nothing destructive. But the yaks would be yaks and it is likely that Thorax's plus one (I'd imagine he brings Pharynx) would cause some trouble as well.