Hey Ranman808,
Welcome to MTo and thanks for your query
EDIT: Just seen you have been a member here for quite a while - But are an occasional poster - So - Nice to see you again!!
We have very few guidelines or rules here at MT.o
And regarding this thread and its clubs - These are just the clubs (threads with pictures) that members here have started up.
ReamerPunch did an amazing job of collating all the clubs in the first place into the OP - and since he did that, we have added to the lists as new clubs are created
So if you want to start up a 91mm club
please feel free to do so

The only thing I would say is that we already have clubs for some of the individual 91mm models and being the most common size - I guess there is not so much uniqueness to that family of models. ie The models in the other size clubs (eg 108mm) are rarer so more likely to garner interest!!
But at the end of the day it's just up to you and all our other members
