An insider group is a classification of a pressure group
They are classed by the strategies they use and their status in the government. This method of classification was thought up by Wyn Grant when he decided that the old form of classification (sectional and cause grouping) did not fit with the new face of politics and pressure groups.
A pressure group becomes an insider group when the government recognises them as a legitimate pressure group. They then have access to decision makers (hence why it is desirable) as long as they abide by the rules and don't critisise the government. They often provide research when the government asks for it in thr form of reports. The civil servants will then use these reports to advise their ministers.
There is a distinction between external insiders and insiders within the state apparatus. External insiders are those pressure groups such as the AA whereas insiders within the state apparatus are groups such as the police or the Church of England. The latter group is consulted out of necessity. If a bill is going to affect the police, they are obviuosly consulted with on the matter. However the former type of insider is only consulted as a matter of course. It is either up to them to lobby the appropriate people or the government will consult them if it thinks it might gain some insight into the matter from them. The more influencial these external insider groups are the more likely it is that the government will consult them.
The strategies that the pressure group uses can also be used to classify them as an insider group. Insider groups (due to their position) rarely use direct action since they don't want to upset the government and have their insider status revoked (Greenpeace, on the other hand, does use direct action even though it is an insider group). They use their contacts in the government to lobby over issues and they give information when consulted (and when they are on commitees).
The Confederation of British Industries
The AA (The Automobile Association not Alcoholics Anonymous (yes, they are actually a pressure group))