In just about every level there are sections that require 2-4 people (mostly 2 person, one 3 person, and two 4 person areas) to get "into". They aren't necessary to finish the level, just necessary to get 100% collection rewards.
Some of the items in those areas are a bit cooler? Costume parts, etc. For the most part my 5 year old daughter has been helping me with those areas. She is actually much better at it than my old lady... sheesh.
XI Wiki


