A sectional group is a classification of a pressure group
This sort of pressure group looks after the interests that will benefit a certain section of society such as all the miners or all the drivers in a society
They gain their members from that section of society only. Only miners can join the NUM (National Union of Mineworkers) and only British industries can join the Confederation of British Industries. This restricted membership makes their ability to recruit new members limited to the total number of eligible people.