O.K., since I think I know the answer because I researched it on DIS, I'll do my best to answer. If I'm wrong, please help me correct it.
"Fish Extender" appears to be the "bags" we hook to the "fish" (litterally) on the wall next to the cabin door. We put one opening for every member of the party in the cabin. People who participates in the FE will make/buy gifts to put in other participant's FE.
I don't know how to start a FE group.
There are threads that are VERY explicits with ample pictures of FE and of FE exchange, on DISBoard. I'm still too new to DIS so I don't know how to put the link on this post. But if you search "FE" or "Fish Extender", you should find it very easily.
"Gifts" range from a deck of cards with Disney characters, to homemade bags, to candies that come from Australia or England we cannot find in Canada or US, to paintings, to pens and pencils, to whatever your imagination and your luggages can take! Some people even ship their FE in a special delivery box to the ship! Guess I won't do that, but, if we have a FE for our date, and if you want something special from Canada (like special chocolate bars that don't exist in the States), just let me know and we will be pleased to share.
Some more crafty people will make their FE. Some people buy them from others on the Board. I'm anything but crafty, so I won't make them, but I won't special order neither. Some people make different gifts for different crowds (little girls and boys, teens, etc.)
Because of DD special needs, I researched the DIS to the max and I found FE examples months ago. I already have three little things gathered for the FE if we care to have one. Some of these were bought at Costco for a special Bday and would nicely fit in the FE bag.
Now, if you don't already know it too, many people also decorate their doors with doors magnets. There are very special threads on
DCL doors magnets with extremely talented and generous people making/sharing them for free. So, I also started to collect door magnets. Therefore, if you come to our cabins for the FE, you will also see the special decorations on our cabin door... including the alien green mickey's head (AGMH) I found at Home Depot!
Hope this answers your question about FE and more,
Hope my English is readable,
Tout plein de poussière de fée,
Cilvy (Clochette Nordique)