Split into yes v no, with around 2 or 3 paragraphs for each side of the argument (depending on marks for the question). Yes: they represent minority views (with about 3 different pressure group examples focused on this area), they encourage political participation (with membership statistics, specific examples from websites such as 38 Degrees), and allow for a pluralist system where power is spread out (insider v outsider groups and the benefits to both, the ways in which pressure groups have put pressure on political parties).
No: representation isn't equal across society (not every part of society has a pressure group, and not every minority group is as politically strong as others), cause problems with parliamentary sovereignty (a single pressure group has the ability to block bills if powerful enough, and doesn't represent society as a whole), and pressure groups themselves can be undemocratic (money buys power, unelected representatives, members being very small sections of society).