Okay this is how you play. I'll start us off using Vegito. So Goku and Vegeta are the two components of this fusion.
The next person comments his new fusion involving at least one of the character/components from the last user's Fusion which in this case was Vegito (Goku or Vegeta).
So their job is to get any other random character, fuse it with either Vegeta or Goku and you have your new fusion.