An outsider 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 is called an outsider group when it lacks that inside connections of an insider group. It tends not to rely on working alongside the system to get the results it wants. They use petitions, rallies, media stunts etc. to gain the decision makers' attention. Although an outsider group will lobby people (both civil servants and ministers) they will not always know extensive groups of people or have access to extensive groups of people in the government. They often use direct action as well as indirect action which gives them a large range of tactics to use to influence government. The group may be invited to take part in consultations and commitees but with less regularity. Research from these groups tends to be looked at by the government.
You also tend to get NIMBY groups classified as an outsider group. This is because they tend to be very specific to one issue and so cannot work their way into government to lobby hard etc.. They can, however, use the influence of local people on the decision makers to get change.
The strategies that the pressure group uses also classify them as an outsider group. Outsider groups don't need to worry about their status being revoked since they have none. They often use direct action to influence government policy but they don't use it exclusively (Greenpeace, for example, does use direct action but also uses extensive lobbying). Outsider groups often rely on the media to create public awareness and sympathy towards their group. If they can evoke sympathy towards their cause there is more chance of the government taking notice of them (they can't ignore someone who is popular).
AAL - Anti Abortion League
Anti GM (Genetically Modified) crop protestors are also a famous example in the UK. As a media stunt they dressed up in bio contamination suits and went around GM sites destroying the crop. Although they did little to actually remove GM crops from the face of the Earth literally, they did gain a great amount of media coverage for themselves. This brought their issue (and their argument) into the limelight.