There isn't really an official position thingy about specialist groups.
Atm these specialist groups mean nothing, as most of the leaders are inactive, or were picked at random

Also there is no point really joining one currently, they don't do anything, it's just a title (such as Mining Expert).
If the clan becomes more active and organised then maybe sometime this system will work, but currently it does not.
The ideal would be that the leaders of each group would organise skills trips, such as one per week.
For example the fishing specialists would meet up, and fish a few 100 fish between them. The rank of your skill would maybe determine how many to fish. These would be turned over to cooking specialists (uncooked), and cooked by those