If our group size increases (from 2 to 4 people), would we be able to buy two more tickets and add them to our group so we can all sit together?

They didn't assign our seating until we arrived. As long as you all arrive together, it shouldn't be an issue.

Ask when you first walk in. They should be able to allow for it

Most likely yes, pending how crowded things are & your package selection. It should be no problem though

No assigned seats. First come first serve, just tell them when you arrive.