A number of egg missions require you to simply join a teammate in order to benefit.
e.g. Defeat # bases or Destroy # Towers.
The team mate can do 100% of the work and it counts towards your mission progress.
Other missions require you to do the work.
e.g. Deal # damage with X dragon
Q: Does anyone think this odd? If your mission is to destroy X bases with Draco and your teammate does 100% of the work with draco and without swapping… Should you benefit?