The forum rule isn’t really specific on this. Does Sunny have to be the star of the story in order to put a fic in the group or can she be in it as a supporting role? Same question goes with sister groups for the other g5 ponies.

7591101
No, she doesn't have to be the main character of your story.
Generally, if Sunny is apparent in the story, then feel free to add! Same goes for other groups. :)

7591105
Okie dokie lokie!